If I answer this honestly I'll be kicked off this forum....
Ah well: I love giving gifts. I think kite has the best philosophy posted so far, and the Amish thing is nice.
But I am probably in the 'extreme' gift giving category. I once sent a couple to Hawaii, all expenses paid. (and....I've never been there! but they needed to go due to some trauma, I had the money, I sent 'em).
I'm not 'always' like that. But just like when you're writing, and there is 'one' perfect word to express something...at any given time there is one perfect gift for someone. Something that fits them, that delights them, that speaks to their core being, that makes their eyes light up with joy, etc.
Finding that, and being able to share that with them is awesome. Doesn't have to be expensive: went to a friend's church once for her birthday. She's always trying to convert me (never gonna happen) but it made her so happy that I did that, it was worth it to me.
I only have a few people I gift regularly though now my Dad has passed on. But they're always something that delights the person.
Worth it, to me.
As for the OP questions: the housewarming was fine, whatever you decide to do with siblings is fine. The 529 is problematic because even though they are now wanting to change from your original agreement, it's still a gift. So I'd let it go. But if I gave money for a certain purpose and felt it wasn't being used for that: I'd stop.
Years ago I was sending money to Tibet to supposedly help with the care of orphans...the 'child' was supposed to be writing me letters but the letters would have stuff in them like "when the Chinese invaded life became hard for me". And the fact that it was obviously an older adult writing pissed me off: a ten year old kid can write, or draw a pic, or whatever. I quit sending the money.
Like SO many things on this board, and in life, you really have to decide what you believe in and what matters to you. And act accordingly.