that is your opinion, rightly or wronngly and I know how you feel.
But unfortunately as much as you might try to stop it, she will resist even more.
Best advice is be aware of anything your daughter might say to you, at the end of the day if the kids are happy then thats best, but if your daughter has concerns she may tell you, then its time to act.
There is no law that states that mixed siblings shouldn''t share a bedroom, the Housing Act 1985 s(328) makes provision under the rules of over-crowding for mixed siblings to no longer share a bedroom when one of them is aged 10 or above.
Can you suggest an alternative to the current arrangements at your ex''s house?