My golden retriever's mum is being mated again this season and if successful the pups would be ready for homing about mid May.
My girl will be 22 months by then and is pretty well behaved. The puppy would be a full sibling.
What I'm interested to know is whether it would be better to have a girl or a boy should we decide to have one of the pups.
Bonnie is a very calm, submissive, loving girl and obviously I want to do the right thing in order to keep her this way!

Is your bitch spayed? I only ask because I am living in h*ll just now. I have a bitch in season, her daughter is thinking about coming in, and I have an entire male in the house ;-) Not ideal, I admit, but we are coping. Just! It wouldn't matter to me if he did catch either(once enough time has passed!) as I originally brought 'mum' into my household to mate with him anyway, but it's something you wouldn't want from full siblings, is it? If your bitch isn't spayed I would have another bitch. If she is then I don't suppose it would really matter.

She's not spayed but I will possibly have her done mid season (not long had 2nd season) as I'm not going to breed from her.

Then I would say get a male.  there is just a chance that with her being quite Young and submissive her younger sister could be the opposite and with a smallish age gap there might be some hierarchy issues with younger one wanting to be boss, but opposite sexes rarely have these issues.

Is it often recommended that very submissive bitches have a dog in the house? I had wondered this myself for when the time comes.

No it is just a very submissive older bitch may not engender respect in a new bitch pup so that eventually, probably before she is mentally mature enough the pup wants to lord it over the older wimpy one, and you could have problems with fighting.

Generally a dog and bitch pair (as long as one is neutered) are least likely to have issues of rank.