Uneeq: Did you not contradict yourself? Also it seems the OP is looking to improve her relationship with her in-laws, not destroy it. We are here to build ahavas yisroel, not sinas yisroel.
MorahRach: It always helps me to recognize that everything is meant to be is a nisayon. Everything that happens in our lives is Hashem speaking to us and helping us build our characters. Hashem hides his nisyonos through nature, so although the other person may not be making the right choice, Hashem did allow you be the recipient of their unfairness, because for whatever reason He decided, YOU apparently need this test. If they didn’t have the money and if they didn’t give to the others, it wouldn’t be a test for you. It is especially at a time when someone else is ridiculously unfair to the extent that it isn’t sensible, that we realize that it HAS to be from Hashem.
This not to say that a person can use this argument to harm others ‘to help G-d’. Proof is ‘migalgilin zechus al yedai zakai vechova al yedai chayav’ (so with that argument that you are ‘helping G-d give out nisyonos’ that would make one chayav).
Remember that it is all a test of G-d seeing if this will help you come closer or further to Him and His commandments. I hope that helps.