My sis in law is a compulsive liar and spreads lies about me i ignore it and try to act bigger then that,?

but she is really annoying me the other day i made the mistake of asking my partner to have a word after yet another nasty comment she then deniyed it, her mother said i didnt hear her n now im the one thats made out to be the bad guy n the liar, i have to deal with these people because i have there granddaughter,and my partner doesnt believe me because in his words its his sister she wouldnt say/do that i guess what im asking is should i stay with my partner n carry on ignoring there insults or just give up on the relationship for the easier life,