i don't know man. on one hand ya she is an adult but at the same time she's 18, that's still a kid in my book. he's 35, her actual blood uncle, and this might have started way earlier than her being 18 and technically an adult which would mean he could have possibly took advantage of her.

this whole situation might blow up, especially with the way these two are going. it almost did during that family gathering earlier this year and with them still arguing and kissing out in the open at the beach (no one was around, but it's in the open someone could have seen just like you did). more of these situations and ya things will definitely come out and blow up. if family finds out you knew about this even earlier, then you're feeling their wrath too.

again, they are technically adults but again she's still young. personally i'd speak to her and then depending on how that goes tell your parents. imagine they find out and how sick they'll feel themselves by bringing them together during all these family get togethers.