Wow! This completely reminds me of a situation I myself was in just a couple of years ago. Let me give you some context. I have 8 female first cousins, all of us in the same age range (now all in our 20s). At every gathering or event some random Aunty would say (in front of the others) that I am the prettiest (dumb thing to do I know but who can question the pakistani auntys). Add to that a fabulous career and other things (Allhamdullilah) and it was inevitable that the other girls would begin to dislike me and behave in a similar manner to your cousin. Except I am not the type to stay quiet and it would always result in a full fledged argument. To make a long story short, I am now no longer on speaking terms with any of them. With my male cousins, yes. But not my female cousins. We haven’t spoken in about 2 years (yes they live nearby). My mother was disappointed at first but now she understands and accepts that it is what it is. I didn’t invite any of them to my nikkah and they won’t be invited to my wedding. I have no patience for people such as this. I am not recommending that you take this course of action because I was dealing with severe bullying on their part, however, I would encourage you to stick up for yourself in front of this particular cousin. My mother never used to understand in the beginning either and would get mad at me for arguing with them but it all comes to light eventually. We simply ignore one another now at any events and they mean nothing to me.