I wonder how my mom, uncle, etc. would react when I tell them I'm aromantic. Honestly, I feel like that would make them shocked. Thety are very accepting, like if I was gay then they would still accept me. But not wanting anything I feel would make their eyes wander.
Subtle tips probably won't help that much. I'm sure you could just tell your parents and ask them to not tell your grandparents if you are worried about them. That is what I did in my case, just told a few people who I knew would not hate me and ask them to not tell them about it.