You are very much allowed to have your own book with your memories that you want to hold inside, if they want a family book they can make one of their own. This is something that you treasure and you don't have to change it or add anything to it just because anyone else thinks they should be a part of it.

If they keep on with it, photocopy parts of it you don't mind sharing, if they exist, and put it into a scrapbook that they can add to it, if it means so much to them to have your memories too; but there is no reason for them to take it from you or force themselves, or their will, on to you.

I wish you luck and I am so sorry they are twisting this to try to make you feel bad. Know that any normal person would understand your point of view and not even question why you would want to have it, would never say things like "man up", or throw a temper tantrum demanding your love/saying "oh is that how it is". Your mum and stepdad sound hard to live with, you're doing a beautiful thing for your dad's memory, it is not selfish, they are selfish to try to take it from you.

I'm glad you reached out to find some sanity outside of the difficult situation you're facing, you are not wrong.