I’ve never tried to fuck a family member OP, so I don’t know how relevant my advice might be, but nothing is ever going to happen if you only see her every couple of years.
That sounds like 1) you’d never get any one on one time and 2) she’d never shake off the little fat kid mental image she probably has of you. I have younger cousins who I still think of as kids even though they’re pushing 30 lol
So if you’re serious about this, find a excuse to be in her presence more often. Maybe take up a new hobby or class that’s in their neighbourhood, then offer to drop in after. Does she have kids? You could start hanging out with your cousin more.
After you establish rapport, just treat her like any other woman, not your aunt. Allow her to see you as a sexual being. Tell her about your Tinder adventures, your ex girlfriends, your friends, your job, your fitness activities, your ambitions. You want her to see you as a man, not a boy.
Signal sexual interest while giving yourself enough plausible deniability to cover you if she doesn’t reciprocate. For example, if she does the aunt thing of “you’re so handsome, why are you still single?”, you respond “If only more girls were like you, Tía” Or ask her if all women have a type. Doesn’t matter what she responds, you swing the conversation over to what your type is and basically describe her “I’ve always liked girls with caramel skin and dark curls.”
Break the touch barrier. Hug her hello, then let your hands linger on her for a fraction of a second. Kiss her on the cheek for hello and goodbye, but plant your kiss slightly closer to her lips - not so close that you’ll kiss her lips, but not in the middle of the cheek either. Put your hand on her waist or the small of her back when you’re brushing past her to get something.
Finally, introduce some alcohol into the mix. Find an excuse to drink with her - not a lot - 1 one drink will do. Get her to loosen up around you and enjoy yourself.
