Child falling asleep in our bed

My SO has a daughter from his previous relationship. She is 5. We live at his parents house right now and daughter has a bed out in the living room and sleeps out there with my MIL. She has her own little spot in the corner behind the couch so she has some privacy. We are planning to move her into my SIL room with her so she can have a room to sleep in plus SIL is never there anyway. Daughter never had a problem going to sleep in her bed and was always fine because she had grandma(MIL) out there with her. For the past 6 months she has been choosing to fall asleep in our bed and we put her in her bed after she is asleep. She said its because she doesn't like being alone when she sleeps. The thing is she's not alone out in the living room because my MIL is out there. About 6 months ago when this started happening her mom filed for full custody and had told daughter she needs to complain about having a new bed and that she really didnt have to come over if she didnt want to. Her mother also took away our overnight visits during the week so we only get her over night every other weekend(no custody has been set up yet through court and we are waiting for a date still). We plan on getting her a new bed when we move her into a bedroom but for now we dont have the money and we cannot put a bigger bed in the living room. She still fits in her toddler bed and has a good foot left before she grows out of it because she is such a tiny girl but her mother is telling her only babies sleep in those beds. I really feel this is the reason she is choosing to sleep in our bed instead of hers. Also in the past 6 months daughter has become more attached to her dad and i than she used to be. Her behavior has completely changed ever since her mom filed for custody. Im wondering if we should let daughter do this or if we should try to get her to fall asleep in her bed. Also could it be daughter is doing this because of all the changes her mom has made in the past 6 months and the things she is having to deal with at her moms that she would feel more comfortable being with us and falling asleep with us around?