I sort of agree with Eve here.
You mentioned the ex cheated. Did they break up because of that and did your partner actually got over the whole thing or now all of sudden there is like well the ex is better now, remorseful, some wishful thinking?
At the end of the day, this is what I recommend for anyone who is dating a person who has one ex or many exes around. If the exes are not getting their own lives and are not moving on, there is some unfinished business and you are not crazy to sniff it. This does not mean hanky panky is going on. This just means, there is lack of maturity and awareness of time and energy for the new relationship.
Just imagine how much time you are talking , thinking and worrying about your partner (not sure the sex you were vague about this and makes it hard to write a pronoun) and their ex and you are not even connecting, being intimate and being in love and HAVING YOUR OWN LIFE!
Drama begets drama. You can get to know the ex, invite her and have fun and create a real friendship or stay in the sideline being angry or just get your own life going and see for this relationship for what it is.
You cannot change others. You can only change how you react and feel and all these feelings sound like you are not living your life the way you wanted and you prefer to put the blame on the other person.