yep. if this isn't out of character then you gotta suck his D or do something nice, but if this came out of left field i'd be a bit skeptical. maybe he's just a nice guy though haha
Nothing...he's boning your sister. Or!!! Plot twist? Are you boning his sister? If so we need a new thread in your appreciation.
I've got 29.5 years left of a mortgage to pay off and my brother-in-law hasn't given me s***! Seriously though a good ol' fashioned should even things out a bit between you two.
Rather than buying him something, you should instead make yourself extra available the next time he needs help with something. Maybe if he has a medical issue or if your sister has a baby, you should offer to go over to their place and get groceries or do housekeeping. Or be the guy he can call - no matter what time- he can call to pick him up when his car breaks down or if he just needs to rant on something. Be willing to inconvenience yourself beyond what you normally would in order to help him out. I think that would be the best way to repay him.
No need to do anything for him. He did a great kindness and, I suspect, is not expecting anything in return.
Save that 8100. (plus whatever you already have saved) Down payment on a house. Subtle reminder of his "gift" 6 months later.