I have a dog of 10 (castrated) and two bitches (mother and daughter) 6 and 4.   The three of them have always got on very well.  Last week my 4 year old bitch attacked the 10 year old male over an incident involving me not letting her follow me out of a door.     It was quite a nasty fight which I broke up.  All settled down the next day.  No problems till today.  I walked through the same door with the dog and the four year old bitch went for him again.  Harder to break up this time and the bitch is now livid.  She does like my attention and is very needy.  She’s half way through her season.  Earlier today all three were sleeping on me.   I’ve never come across a bitch going for a dog before and so viciously.

I only have bitches, but they can be a lot worse in an altercation, thankfully I've only had a couple between them.  Most dogs back down with a bitch, but it just depends on the situation.  If this is likely to continue to be honest you may have to look at rehoming the older boy if possible, or at least working out a routine where no situation occurs that could cause anything to flare up between them.

If this going through a door is a trigger point with her I would make sure that she is split from this dog for the rest of her season and see once hormones start to balance out again if she is the same. I would put management in for trigger points so she can't do this to your boy

> She’s half way through her season.

There's your answer right there!!  Why is she with the others at this time, even if the male is castrated?   It would, in my opinion, be worse had she fought with her mother because all too often, when bitches fight there's no way back. 

However, obviously you need to avoid the same situation with the door from happening again.   Whether this will continue post-season is anybody's guess, but for now just remove her from the other two until she's out of season and then, if she were mine, I'd plan on getting her spayed midway between this season and the next.  It may be that you'll have to do that, and if she doesn't quieten down, find a good home for her so your poor old boy can live out his days in peace.

Thank you for your replies.  She is NOT in season.  My mistake!   I meant to say she is mid way through her seasons so it’s nothing to do with that.