Hi, i grew up as part of a 'step' family, but it was kinda diff as i was younger when my mum and dad split, and she re married. And i did not see my real dad for many reasons, Know i am 22, i see my real dad as not my dad, as my step dad was the one who was there through me growing up.
If it was now, i think i would find it tough, but if it made my mum happy then i suppose i would just have too deal with it.
You said you are scared he will hurt her, why
Has he done anything that makes you think he would?
Listen i know its tough, but for your mums sake, just let it be. She has too make her own choices and if it's what she wants, then i am afraid its going too happen. If you really dis like him, you just have too grit your teeth. You never know it may be all okay! Which i am sure you would want it too be!
As for him ''playing'' dad/grandad then just let him, he can be there for you and your child, as a ''father/grandfather'' figure can't he?
He knows he is not your real father, but just give him a chance. Let your child know if/when they are old enough who he is, but whats the harm in him being a grandfather figure, surely its a good thing, growing up with lots of family is great!!
And if ( which i really hope it does not!) turn sour, be there for your mum.
Just let her know you support what ever she does and you love her.
And just let things happen. I am sure all your worry is bothering you. But i really hope it all works out well for you all.
nikita x