Hi really not sure how to  go about rwriting this so hopefully it will make sense whilst I Type...  

Right firstly  I had my friends mastiff staying with me I also had one from the same litter. Theyre both 10 months old Now. Shed been staying with me because her dog had to have a op on his knee as he had knee dysplasia so was on total bed rest . i had her pup for over 2 months .  Both were females .
Anyways we had a female dog being a jack russell and a Great dane male.  The female JR was very dominant and  my mastiff when she  had a cob on would not back down.

Okay yes they had squabbles every now and then as most dogs do but would back down after a few growls and  snaps .
When we went out we left the  gang together as we always did - Altho my son was at home  - he couldnt manage to seperate them - altho tbf  i wouldnt have expected him to as hed of got hurt himself.
but this one was  went to far and sadly the mastifs killed the jack russell :(   Im not 100% sure what happened what caused it or what wether it was pecking orders or pushing levels. or if something caused it I dont know :(    I had to let my friend take hers bk she  rehomed her instanlty and mine went to my parents.

In that time I wanted her back missed her so much from having a house full of pets to having just one :(  It was to quite and as she  never showed agression to me took her back in .  Despite alot of arguments from people and my other half saying Im crazy taking her back  as once shes tasted blood she cant be trusted , but I personaly dont see it that way. Yes I have children But she is never left with the smaller children ...

Any ways since her return home shes  well very submissive to the point when she plays with next doors dog (they have one from same litter) she lies down and literally lets the bitch pull her every where where as before this she was the one  doing the playing  shed literally grab her by the  neck all the time... where as now shes not.
Any dog she meets  she is quite ?
Shes very respondant to me to where as before she wouldnt listen...   she was playing with next doors dog  before and the bich had her by her neck and was literally dragging her across the floor where as before the 'fight' she would have  never have been lay down for her?   Is  it because now she doesnt need to push to become the top female?  but why be submissive?  urgh confusing animals!

Is this normal behaviour?   Im confused with her , she is awkward as welll atm with food she will look at it then walk away where as before she dived right in. Now she wont.  Is she pinning for her . i dont know,. a this probably makes  no sense !   but thanks for reading! :)

I'm so sorry for the loss of your dog, especially under those circumstances :(

By 'mastiff' do you mean English Mastiff?
Is your Mastiff 10 months old now, or was she 10 months old when the incident happend - if so how old is she now?

There are a few things that suprise me, firstly, your Mastiff pup getting a 'cob on' with the JRT, they should be a very patient breed and I'm suprised one would get irked by a small dog giving it some attitude, also I;m suprised at a Mastiff allowing another dog to drag it accross the floor by its neck (that is a whopping weight to be pulling around using teeth on flesh without causing damage or ripping or puncturing flesh!!!) and suprised at the sibling playing like that.
That said, from my limited experience play can be rough and without calm,kind patient handling/training I can see how one could be more than a handfull as a youngster, let alone 2 siblings (if I'm reading the post right).

> Shes very respondant to me to where as before she wouldnt listen

I know it's not too wise to judge behaviour over the 'net, but to me this suggests she is lacking in confidance/feeling insecure (IMO a frightened animal is a dangerous animal). If she is only 10 months old, she has been from the breeder, to you, to your mum then back to you, that is 4 homes in her very short life, for a dog that gets very atatched to it's people, also a very sensitive breed that will pick up on the smallest 'bad vibes', especially if she is allready lacking in confidance/feeling insecure.

How long has she been back with you?
Does the breeder know what has happened? Has the breeder given you any advice?

This is a huge, powerfull dog, I strongly advise you to get her to the vet for a full check up & then to see a good behaviourist. You need to know, as much as it's possible to, why she is behaving as she is. This dog potentially has serious dog-dog aggression - the incident with the JRT may be limited to a ranking issue (3 females inthe house) and not to general dog-dog aggression - but you don't know what caused the incident and you can not take chances, especially with a dog this size.
Pelase make sure you have complete physical control of her on walks. If she does have issues with other dogs - she may be hiding at them the moment if she is feeling so insecure she is shutting off from everything, or doing her utmost to 'appease' you.

Please see a behaviourist that can assess her in person, so you know what you are dealing with - it may be nothing, she may just need time to settle back in, but I would NOT be taking the chance with such a huge dog that can out-power you.

Also could the death of the JRT been accidental. 

At 10 months the two were just pups, not fully aware of theri own strength, and if play got out fo hand a small dog could easily have gotten crushed.

Are you experienced enough to distinguish vocal rough play and actual fighting?

> if play got out fo hand a small dog could easily have gotten crushed.

Good point Brainless,
You are right,at only 10 months old they are still very much pups.
Busters play could get rather exuberant at that age (he knocked me to the floor when he was younger than that ), if I had to leave the house I would not leave him loose with my other dog for fear of accidents, so with 2 mastiffs and a JRT, squashing during play is a very strong possibility :(

No death wasnt accidental :(  she had far to many cuts etc :(   The JR was bit through her neck :( 
Both mastiffs were covered in bites. :(  one in the mouth and the others were on the legs. 

The pups are English x  french (dogue de bordeaux)    
My Mastiff when I say cob on I mean  if  one got a  bite in in one place playfully she wouldnt give in . shed carry on etc ... untill the other would back away . but now she dont .. she just lies there?
shes 10 months  now. this happened 6 weeks ago .

Yes  they were two siblings from the moment they got together they were both uncontrollable - I know there only pups but all they wanted to do was play NON stop!  and my house wasnt really ideal to keep them . And as i had the two together walking them was a night mare!  shes actually been in 5 homes before the breeder  she was at my friends as the mother nearly died.  My friend took 2 pups in at 5 weeks old  as  they  would have died.   I wouldnt say she lacks confidence as she is the type  where shes very happy  to meet people not shy - etc.  shes been back with me for 4 and a half weeks now  she was at my fathers approx 1 week and a half near 2 weeks.   But in the time she was there I saw her every day  .  It broke my heart leaving her  and all she did was pine for me :(  But I knew she was fine as she adores my father. 

Now the sister is gone she listens to me completely .

Breeder knows altho wouldnt class her as a breeder more some one who put the dogs together for quick easy money - have since found out that the mum had attacked a male dog it went to be mated with And attacked another  dog.

she has seen a vet who basicly  just told us to be wary as she is a big dog and as Ive kids just dont leave her alone with them , but cant obv guarentee if she will change but did state she thought was pecking order gone to far :(  Shes mixed with other dogs since   Im walking her as much as i possibly can and taking her everywhere   so she is socilaised more and more!  I have control of her completely she walks beside me on her lead and have her on a harness to i have full control of her .

would def be interested in a behaviourist ? can any one recomend one?