If your brother seems to be the favourite in regards to the kids - can you think of a reason this might be?
I'm only asking. But ask yourself whether you were dropping your kids off with a list of does and don'ts? Special diets, only eating homecooked or grilling them about minor details, texting for updates, specifying naps, baby can't do xyz etc? Or do you have too much of a laid back approach vs your parents in terms of - we don't say 'no', we talk about gentle hands which your parents might be eye-rolling at etc etc
Were you frequently late when picking up? Did you show appreciation with meals out, or invites round, vouchers, the odd bottle of wine given or other financial contributions?
When my Mum has had this between my two brothers and their kids/families - there's been a reason her side (even if she didnt disclose it except to me). It was a, 'I don't want to offer to X because last time they did Y and I feel so unappreciated.'
It was never about the kids - always about how my brothers and wives treated her that led to periods of time when she was more for helping one than the other and to be honest, alot of the time, I could see her point.
My brothers though never recognised it in themselves. I suppose you wouldn't unless pointed at.
The other point could be your brother needs the help more than you - maybe for financial reasons, or possibly because a full time spot n nursery wasn't available for them. Could be they don't want to be doing it, but are because your brothers in a pinch.