Why was your brother having to see him in a contact centre?
You've had no relationship with him yourself for 14 years it's not fair to just land on him and make him have to deal with wondering how to respond to a gift from an aunt he doesn't even remember.
You are very negatively judging her when your brother basically abandoned her and his son at the very least - I suspect there's more to it with mention of a contact centre.
As pps have said, she's the parent that's done the hard graft.
You also don't know if the money was spent on the holidays, if the money from the holidays came from elsewhere or if the son himself wanted the money spent that way. You say it was only a few thousand I'd be very surprised if that was even enough for more than one holiday for a family of 4.
Ensure she has contact info of yours to give him when he's 18 if he wants and if she thinks it appropriate.
I really don't think doing more than that would be appropriate.
"but it does stick in the throat a wee bit that she has hardly ever worked in her entire life but can afford three holidays a year, various weekends away and lots of meals out." Why? It's none of your business, you don't know her new husbands situation so you don't know where the money is coming from. If it's bugging you that much stop looking.
"You have no idea that this doesn't apply here. You've not seen her in 14 years and were never close." Exactly. There's no way you can know the details of her finances.