Partner too close with cousin

I know this is going to sound ridiculous and irrational , but I am uncomfortable/ jealous with the relationship my partner has with his female cousin she is young and pretty and they get on so well , she is basically his type (based on his ex's) and they have made dubsmash videos together acting like a couple n stuff and they're really close he always goes on about her and how pretty she is and people tease them about going out because they make a good looking couple etc n I was like wtf they're cousins he has assured me he wouldn't marry his cousin or whatever , There are boundaries and they cross them being touchy-feely and constantly complimenting each other ,  am I being irrational ?