Like the majority of everyone else that's answered, I think you're putting too much negative emotion into something that's not a big deal.
Any germs that the young boy would be leaving in the bed are there to be spread on toys, the couch, carpet, the air, and pretty much the whole rest of the house. Let's not forget that your daughter has her own to out too. Also, germs are not these evil, children hating, things out with a mission to destroy. Exposure to everyday germs early on enhances one's immune system.
The only way I see for annoyance is if you purchased the bed yourself for the sole intent for her to have a bed at grandma's, making it 100% hers.
You have every right to be annoyed about the food and diapers, if the ones you're providing for your daughter are being used for the boy.