How about a quid pro quo? Show up at the funeral in boy-mode, and make it clear that's the last time you ever do that, and come out to the family within the next month.
Is 19 really too young to drink at a family wake? A Catholic family? I mean, if it were Baptist or Muslim or Mormon, that would be one thing. But Catholic? Seems odd.
Clothes: black blazer, slacks, sweater*, and women's oxfords, or something along those lines; hair down. And lay low, keep quiet, and let people draw their own conclusions. Hey, it's a funeral, you can wear black. Alternately, do the fitted shirt thing, wear a thin black tie, and tell her to pretend she's Kate Moennig. Every time a relative says, "You're looking very ---," mentally replace "---" with "Shane."
FWIW, I had to spend a whole bloody weekend in boy mode with the family last fall after being full-time for months, and while it really, really sucked, it was nevertheless a good learning experience that made it really clear I would never do that again.
* (or whatever; I wouldn't go for the button-down, because that would necessitate a tie, and that would be hell. Ties are lovely -- on anyone but me, say I. You could go for a women's plain black mock turtleneck, and it wouldn't matter to anyone else.)