What's good for the goose is good for the gander. I think it's VERY important that your child recognize that the "rules" are the same for both parents.
That being said, I think you should (if you haven't already) plug in "Why kid's should not sleep in their parent's bed" and look into what Psychologists are saying on the topic. Prolonged sleeping in a parent's bed have far reaching effects into a young child's life,i.e. fostering independence, confidence to sleep soundly by themselves, inability to nap during the day without the presence of a parent etc. There are many good articles on the subject, "The pros and cons of co-sleeping with your child" "The impact of co-sleeping with your child".
When i was in the same position as you, having to share a room in my mother's house with my 5 year old, I set up a bunk bed, with a desk underneath, and put my single bed behind the bunk bed. It was a tight fit, but it felt like my son had his own room and he felt special that he had his own bed and desk and place to call his own. Still, I remember my son staying awake and asking me when i was "coming to bed". His worrying about my not getting enough sleep was affecting his sleep patterns. It was not an ideal situation, and most significantly, he was unable to reciprocate having "sleepovers" like the rest of his young buddies.
I too think that your exes partner will help to move his and your daughter out of their bed and into her own, but your situation is more difficult to manage, but it might help to consider using bunk beds so that your daughter feels her own autonomy and has a more normal experience at bed time and less chance of experiencing the negative affects of co-sleeping with a parent.(not the least of which is feeling that she is being replaced at bedtime, if and when you find a partner in the future).
I wish you all the best in the new year, and please know that you always have friends here to talk to privately, in case you need a sounding board.
I think it is okay. She's young. They will not be having sex with each other when she sleeps with them. And most likely, the new wife is determined to get his child out of their bed as soon as possible. (most new partners really don't like the kids sleeping with them . . . ). She's in a transitional time if her parents broke up, dad has a new person in his life (maybe there are other kids around too or on the way?)--- and this is security for her.
She sleeps with you . . . that is comforting now as it is her habit to sleep with someone. She probably really wants to sleep with her dad. I'd not get involved. If it goes on for years, then ya. It needs to end. But right now, it's not harmful.
Other than to your spirit as I'm sure this upsets you and hurts you. I'm sorry he kicked you out.
Try not to make wars with her dad. That will only complicate coparenting. And she needs dad in her life, so keep the peace whenever you can. good luck