Hi Taymum, I have a stepdaughter and stepson, I'm also lucky as their mum and I get on well, we did have a bad spell when my son was born but thats water under the bridge and totally forgotten about.
I have never tried to do the motherly bit with them as they have a great mum, I have always been honest with them that although I expect certain rules in my house but they are only here for a short time and just to view me as a friend, I'm not trying to be their mother.
I get on well with my stepdaughter as like you said we do the girly things and we can talk about anything, she knows she can tell me things and I can give her advice adult to child and not parent to child iykwim, I remember back when they were just small and I captured her heart by always having arts and crafts stuff that she could make stuff with, I would always say 'Why don't you make a card for your mum' I have never made her aware of any bad feelings between her mum and I when we were going through the bad patch as at the end of the day she is still the mum.
With my stepson, he was a different ballgame, he is very much for his mum and was always homesick when he came to us, I tread very carefully with him, my hubby spent a lot of time with him, doing father/son things, I also encouraged my ss to ring his mum whenever he wanted, to make a card of picture for her or let him be alone if he so wished, he's nearly 11 now and so much more outgoing, he's brilliant with his little brother and is besotted with his baby sister, in fact when we had them just after she was born he was the first to go over and pick her up when she started crying, he is a brilliant young man and he even told me he loved me when he was last up, now i remember when he was my sons age and he wouldn't even look at me, he never wanted to do anything with me and was always crying for his mum (not meant in a nasty way)
What I'm trying to say, but going the long way around it) is that your ss is still a baby himself and I guess its hard for him too, just be yourself with him, offer your friendship, or ask him if he would like to do something with you and your dd, he will prob say no a lot of the time but then just say 'oh thats ok, but if you decide you want to join us then you come right in' or something like that. It's a long, hard process to win their trust and love but so worth it in the end, I remember hearing someone refer to their stepchildren as their bonus children and thats how I like to see mine as.
If you ever need someone to rant or chat to, just pm me and I'll try to help xx