There Is a diffrence between rubbing against some one, and someone feeling it during a hug... That being said uncle should have did the one armed sideways hug

Yeah, if a guy is hugging a family member, not their significant other, their waist areas don't need to be touching. Honestly when I hug family, it's always a lean in hug, especially if I'm hugging a Male. I don't even hug other girls standing fully straight, so even if a guy has an unintentional erection, I'll most likely not notice unless I look down... Like... idk all the inner workings of the male mind to know why they have unintentional erections, must suck, but if you're a guy, learn to do a leaning hug at least.

She greeted him with a hug. It may have been entirely her pushing into him.