sometimes children really want to share with someone else, chances are that when they get older they will want their own space, as for sexual abuse this can happen when the children do have beds of their own.
The important thing is to listen to your children and keep communication open, and if you sense that one is starting to be not happy with it then you know. Also parent them to sleep and look in on them to make sure all is ok. Obviously if you suspect anything inapproriate then separate them, but if they are trying to ensure that they can tell outsiders that they know what is going on they could put a monitor in the room so they could hear what is going on, obvioiusly hidden [but that is another issue isn't it]
It could be wise to have a bed that the girl could use as her own if she decided she wanted to use it and make it all nice and decorated to her taste whatever that is, and chances are that it will happen naturally
If one of the children still wants to cuddle up when the other one doesn't, then maybe the one who still wants to snuggle could go in with mum