Hale Koa might not be the best idea, if BIL doesn't have an ID. We were there a few years ago and I think they did 100% ID check. And you KNOW they won't feel right about going out without BIL and SIL, so I'd skip the Hale Koa.
Honestly, I'd donate in their name. And not tell them how much. Then you've made the gesture and taken the high road, because yes, as everyone has pointed out, they have probably "done" Oahu already and anything you get is going to be snarked about.
Parents are weird (and I'm a parent) in the way they treat their kids. I treat mine equally (HAH! I think I do) but in 15 years they will probably say I favored one over the other. My parents give my sister money but would never think to give me any...and I'd never think to ask, but my sister asks with no problem at all. I'd be embarrassed but she's not. Go figure.
So they like BIL's family better. OK, whatever. I wouldn't spend any more time or effort on them.
We practically ignore DH's side of the family, because they are pretty wrapped up in themselves and being on the move for 20 years in the military hasn't helped the situation. These are people who have never been outside their hometown. So we are closer with my family, which suits me fine (of course!) and it's reciprocated. We tried for a long time with DH's dad, mom, sister, but just got nothing in return, so we're cordial but don't invest any emotion, time, or money in it.
And as to the comment "the people who gave your husband life"....wow. Uh, not all parents are good parents, and not all people who "gave life" are parents. Just because they procreated and now ignore your DH doesn't mean you need to grovel.