With MY DH and HIS siblings/parents - I couldn't fathom being 'furious'. I'd just wonder why he'd not feel he could tell me. Him and his brother recently bought FIL a zero turn lawn mower and he called me when they were on the way home with it, so it was already purchased (about $5k each). I was not fazed at all, FIL does a LOT for us and it was the least we could do in return.
However, if he'd gave away a large sum to a sibling to p!ssed it against the wall time and time again, yeah I'd start to get annoyed if he did it more than once.
Side note - I lent my brother $300 recently to fix his car. He's paying me back but I didn't think to tell DH about it. I think that is different.