Is there an appropriate time to stop helping a sibling?

My boyfriend has issues with his sister and her family at times. She is engaged and has 3 kids and one of them were just born recently. The problem is she has no job, no car(because she wrecked it on purpose), and her fiance has a seasonal-unstable job. My bf and his mom (they both have full time jobs) has reached out to them often BUT she has wrecked her car as well and her kids broke my bf's truck dash and cracked the screen of an iPad that I bought him for Christmas. My bf says she doesn't try and make it up to them and just dusts it off.
This makes me not want to buy the things that I want to for my bf and the whole situation will put him in a bad mood at times.

If you were in my boyfriend's shoes what would you do?

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