ok a couple of thoughts

I'd probably sit her down and explain that you have issues with not being asked if he could stay over. I'd also explain that you do feel uncomfortable about them sleeping together under your roof, but you're aware that both sets of parents allow it. Let them know your reasons why you're uncomfortable or have issues, let them know that you don't want them to feel uncomfortable in your house or to stop visiting. You may find that they respect your view of this and either not sleep in the same bed or he may not sleep over.

Good luck with it all.

My mother wouldn't let my boyfriend's stay over even when I moved back home in my 20s. She was also horrified when she found out her son (who was in his LATE 20s) was living with his now wife (they've been married 34 years this year), when they were in the UK, before they got married.