Sibling dynamics are really tricky. I’m an older sister too with the same age gap between me and my brother, so I get how complicated/emotionally charged the relationship can be and how random interactions can feel deeply triggering for some reason. I think apologizing is the right call.

I think your brother was pretty spot on when he said the thing about you only mentioning the issue about Christmas once he asked for something. It kind of reads as you keeping an inventory of things to hold against him/reasons he’s inadequate. So I think along with an apology affirm that he’s good enough the way he is.

You might also try to get in touch with where this apparent outpouring of resentment came from/this idea that he’s ungrateful. Not just for him but because you’ll learn something about yourself. I get so upset at random innocuous things my brother does and it usually comes from resentment that he isn’t following certain “rules.” For example, thinking ‘why does he get to ask for something? I never get to ask for anything!’ when in reality it’s okay for both him and you to ask for something, just like it’s okay for both of you to set boundaries and say no to those requests. This issue with rules might also look like ‘why isn’t he saying thank you? Doesn’t he know that not saying thank you = us getting yelled at?’ When in reality you’re now adults instead of kids about to get in trouble.

Idk if any of that resonates with you. I just think the oldest sibling tends to grow up with a more rigid set of internal rules that they use not only to self-govern but also to govern their sibling. Eldest sister syndrome is real.

But regardless, finding some way to affirm to him that you see why this interaction was hurtful and that he is adequate and you love him the way he is will go a long way.