Difficult situation..
My partner is still very, very close with his ex (who is also his step sister!) very odd situation. I struggle to deal with the fact that she is always going to be part of his life, as she is family she is always going to be there. They both say that what happened between them is over, but they still talk almost every day and tell eachother everything. I hate it as i dont fully trust him with her. i love him and want our relationship to work, but i dont know how i can come to terms with the fact that she is always going to be a big part of his life.


Comments
This comes down to trust from your end. what you need to ask yourself initially is has he given you any reason to consider that he may be unfaithful? If he hasnt then reassure your own confidence with that initially. Add onto the fact that she's his step sister now would suggest even more so that he won't try anything as the family side would light up with a lot of negativity if something like that happened (assuming it would).
I'm in a similar position myself where my partner still actively talks and meets up from time to time with her ex. I too also find very hard and have found myself looking over her texts etc as I wasn't sure either, more so where she has recently become distant with me. For you though, I would say is trust your man and so long as hes shows your relationship the priority and full attention then you shouldn't worry, when in doubt though then be open and honest with him and talk to him about it. Express your worry and let him understand your concern. He might be a bit more responsive going forward.
Hope this helps, don't push him away and keep showing the affection while being honest and open with him, he should understand the position you are in and make appropriate discreet changes so you are happier too. :)