Stepdaughter cries nonstop for her mommy. what should we do?

My step.daughter started spending the nights with us in december. She is 2.5 years old. Her mother still nurses her and sleeps in bed with her. She will not stop either to make the transition for s.d. easier. Anyway EVERY TIME she stays with us she says I want my mommy back crying for at least an hour. We have tried everything, reading, singing, tv, bath, the distractions work for a min and she's back at it ... we don't agree w her sleeping in our bed so we don't do that. It has now been over 4 months and it is still the same. We are at our wits end. Any suggestions would help. We are not sure if it is better for her to just not stay overnight or if we should force her. It's causing stream for her and us, but of course my husband wants to see her regularly... I have a 5 year old son and it sometimes is not AS bad when he is with us, which is most of the time, but it still occurs either way the duration just may not be as long. We have her every tues night, and every Friday night. We do not have court ordered custody they just work it out between themselves.