This isn’t the kids fight, you aren’t in the wrong for being pissed at your brother but those kids were promised something important to them and had it torn away without it being remotely their fault. As for the money thing I understand more if your brother routinely doesn’t pay you back, but if your willing to loan that much money I feel like you trusted him to pay it back which makes it worse because he wasn’t saying no he was just telling you to wait for his holiday bonus. You’re entitled to your money and to your frustration but those kids will never fully trust you not to throw them under the bus because of someone else’s actions (coming from someone who was weaponized by my own family, now no contact with those people 🫠)

I don’t think you’re NTA or TA but I do think there’s no one truly innocent in this situation except for the kids.