I think I'd speak to my grandson in your position. Another contributor said they might not want to sleep together at your house and that is a distinct possibility.
Another possibility however is that they might not come or not come so often or stay for so long if you insist on them sleeping separately and that might disappoint you.
Surely the best thing is to speak to your grandson and tell him that you feel uncomfortable but that it's very important to you to have them to stay and how much you're looking forward to spending time with them? You might find he offers to sleep separately from his girlfriend rather than upset you and then you'll have worried for no reason. Let's hope so. 
What’s a household item that reminds you of your grandma’s house?



