What is the most unselfish thing you could do for a friend or relative, but would require you to bite your tongue a bit?

I have an older sister that values possessions for the sake of having them and you don’t. She started sneaking things out our parents house when they were quite old. She is the oldest, and I guess she feels she has first rights to everything. She doesn’t have much in her life that is happy, including herself. She and I are totally different on the morals scale and have had quite a few nasty disagreements. Almost one of the only things I have is my mothers engagement ring. My sister wants it so bad, it is altering her daily life. I was thinking I should give it to her, to brighten her days.