The problems of step-families! Need some advise please?

Before discussing WITH YOU whether you're okay with his daughter sleeping in your bed, he should have discussed WITH HER whether she was okay with you moving into their home. Even if she and her brother are only there every other week, it is still their home and you moving in should have been discussed with them. Just because they're kids doesn't mean they don't get a say in the situation. This could be a reason.