This thread will get hysterical.
However OP, if you are still reading......
I would always say "trust your instinct." My stepfather is one such creep. My best friend told me about a year ago that she never felt comfortable with him at my house but could never put her finger on why.
He was an emotionally abusive man and would openly leer at women. I saw him rubbing the back of my 17yo niece underneath her clothing, patting her bum and tickling the inside of her thighs. She had never said anything because she didn't know what to say, she knew it wasn't right but he wasn't actually doing anything illegal iykwim? Because I witnessed this I spoke out and was immediately cut off and shunned by members of my own family as they accused me of calling him a pervert.
Their reaction was so over the top I still wonder whether he has actually done something wrong at some point?
What I am trying to say is that sometimes you cannot explain why you feel uncomfortable with someone. When you write it down it appears trivial and there are many people who would tell you to get a grip etc. But you alone are in that situation and there is no denying your feelings.
Some people on here have unkindly said that this is your problem rather than his and that you are discriminating because of the age difference between your mum and him. Trouble is, if they carry on making you feel that this is all in your head and to push your doubts to the back of your mind, they could actually be encouraging a situation that may well be very wrong.
It's dangerous to give such advice in such a situation I feel. We are not there, we do not know these people or you and we cannot possibly understand your feelings.
So, to be ultra cautious I would say to go with your feelings.
This is what I would do:
Sit your dd down and have the discussion with her about keeping herself safe. There is a book called, I think, Sydney the Snail Plays Safe and it covers pretty much everything in there. I told my kids that if anyone touches them in an area that is covered by their underwear, they must tell me at once. It also emphasises in the book about children listening to their feelings and telling someone if they feel uncomfortable for any reason.
I would then speak to BOTH your mother and her partner and explain that you don't feel it is appropriate for your dd to share a bed with them anymore as she is growing up etc, and you would like your wishes to be respected by them.
I think you can also check someone now by contacting your local police authority? Just to put your mind at rest? After all, he does have a lot of contact with your dd and if he was a nursery worker he'd have to have a CRC. I don't really see any difference between some in a professional capacity and someone who you may not know that well, who also has full access to your children. You have every right to ask questions.
Remember that people on this thread are giving advice based only on general knowledge. No-one can step into your shoes and tell you what you are thinking and why you are thinking it. Your child is paramount to you and so as her mother, you obviously want to make sure that she is as safe as possible. That is not hysteria, that is common sense and I'm sure most posters under the same circumstances would do the same.