The world is not full of wonderful people, some are nice, some are alright and some just plain nasty.
Nice or nasty, most marry and become MiLs, Dils, FiLs, SiLs etc etc. It stands to reason that some of these relationships will work and others won't. It is the same at work, some managers/colleagues are great are others are horrid, ditto one's immediate blood family.
I think we spend far too much time dissecting MiL/DiL relations. Like every other relationship some work some do not.
Any relationship that depends on one party trying to 'buy' the other person's favour is doomed to failure. Offer someone the help that you can manage and then get on with your life. It is isn't enough for them, that is their problem not yours. They are more likely to respond positively to you because you keep your self respect than if you grovel to them and try to ingratiate yourself with them.
If it is clear nothing will improve your relationship. Do not waste time trying to do the impossible. There are far more rewarding things to do with your time and money.