If Kuu were buying the gift yes,but Kuu's sister is buying(burning) a gift for her siblings spouse(kuu's husband).In so doing the sister is going beyond what most of us would do IMO.
I can see buying a present for my wifes siblings or her for mine
You just said you wouldn't find it weird if you bought a gift for your spouse's sibling. It's just the reverse situation with ku'us sister and hubby.