Well, unfortunately there is nothing you can do about her talking to his ex. I would be careful or concious of what you tell his sister- if anything. I wouldnt share any details about you and his relationship because the ex can use it against you if she finds out. I would be concerned if he talks to his ex, but the sister- dont worry about it too much. Show her you are a cool girl too, but give it time - she may just need to get used to you and you all may become good friends down the road.
I have just read ‘I Hate His Ex’ by Alex Moore, found it really useful. I now feel able to resolve my relationship problems finally! :)My sisters boyfriend.,