Could you call up your aunt and say, "Mom and I were thinking about visiting. Is there a time we can visit when we won't be disturbing Grandson as he uses your computer? We know he doesn't really enjoy visiting with us....hey, he's a 10 year old boy with a couple of older women. We get I t; no harm, no foul. And you know, sometimes topics come up that aren't really appropriate for him to hear." If Aunt says that you won't bother him, then you can always say, "No thanks, we'll just wait for another time then. While it's nice to see him to say Hi, it's just not a relaxing visit if we have to censor our conversations or hear that he's not happy with us being there."
I think this is a very good way of putting it. I also agree with those posters who have asked if there is anywhere else in the house that you could sit. Perhpas Aunt sees the TV/Computer room as her space, and feels she cannot encroach on the rest of the house. And why does the boy have to use the cmputer in Aunt's room? Is it the only computer in the house.? From what the OP says, he has his own laptop, so why isn't he using it in his own family rooms, or his bedroom?