What is the solution for dealing with the husband's sister who incites discord and jealousy towards the wife, especially when the parents are biased towards her, and what has resulted from that in terms of instigating disputes and insulting the wife?

If the sister is causing problems with the wife due to jealousy, she is in error. One should be patient with her and with the wife. The sister should be advised in a kind manner by someone acceptable to her, and her injustice towards the wife should be explained to her. If she does not stop, she may be forsaken if that would deter her. One should be wise in dealing with family and wife, so as not to display anything that incites problems, and try to extinguish the fire of jealousy with gifts. Understand with your parents with politeness and respect, and beware of any offense towards them, for the right of parents is to be treated with kindness, even if they err.