“Theyr also constantly talking against them when they arent here (to me and my other siblings)My father is not talking to my brotherinlaw and sister.”
IMHO and please be moichel me for expressing it bluntly, your parents’ actions are far worse than the fact that your brother-in-law is (horrors!) manning up and actually supporting his wife on his own. Unless there is much more to this story than you have mentioned, and your BIL is perhaps a nasty, disrespectful, selfish, unfriendly louse, your parents seem to be in need of a good talking to by a sensitive Rov. Your sister and her husband may not be following your family’s strict litvish derech (and not all litvish act as they do, either, btw), but if they are following the Torah life, and are nice people, your parents should be grateful that their daughter is married to a mensch, and not act in a way that will surely cause sholom bayis problems down the line.