Hi T.!
So sorry that this is happening, but coming from a divorced background with stepchildren, I know exactly what you are talking about. No matter what you say or do, YOU MUST REMEMBER THAT YOU HAVE ABSOLUTELY NO CONTROL OVER WHAT GOES ON IN THE house that your daughter is in while with her father. The best thing to do is to NOT SPEAK ILL OF HIM OR HIS NEW WIFE, (it will come back to bite you in the butt if you do) and just tell her and him to for that matter, that what goes on in your house is your business and what goes on in his house, you don't want to know. Why should you know, there is absolutely nothing you can do about it except torture yourself, so it is best to just not know. Also explain to your daughter that she has the freedom to ask or tell you anything, but you reserve the right to answer her questions. Simply for the fact that when you do, then when they (father & step-mother) ask her questions regarding you or your marriage or whatever, she feel obligated to answer. If she doesn't know, she can honestly say, I don't know Dad, I just know Mom is happy.
Divorce is so nasty and parents who choose to use their children as pawns have no clue the detriment they are doing to their own children, so even though he doesn't do right, you be the bigger person and do "right by her". You will reap the benefits, but not till much later. Your daught will see through her father and step-mother and know that you were the "best person, right or the best mother you could have been despite all of their nastiness".
Just be open and honest with your daughter, but try and tell her that what goes on in her home with her father, you cannot control and don't say anything negative about it. Act more like a Psychologist when she begins to tell you things they (father & SM) say, like, how does that make you feel? Things and quesitons like this, and then just be sure and say, well I would not necessarily agree with that topic of conversation, but honey, just remember you Father loves you and and he loves (the name of step-mother too). It is SO HARD, YOU HAVE TO GRIT YOUR TEETH AND BEAR IT, but you gotta do it! Believe me, they know already. Once she gets older, she will know the truth and sometimes do already, kids want to believe the best about their parents.
Good Luck and I will be praying for you!!
G. B.