M younger cousin was in love with me and acted on it.
What did you do when she acted out on her love for you?
I treated her like a sister I never had but maybe I really like her underneath. She's into me because I'm popular and always have a lot of girlfriends.
Even if you like your cousin and she likes you, it is inappropriate to have a romantic relationship between family members. Treat your cousin  like your sister and stay away from anything more than that.
I like to think she's not just some cousin, she sounds like a great girl to me.
You can think your cousin is great without engaging in a romantic relationship with her. It's not okay to encourage romance between family members. Maybe you should tell someone else about her feelings for you.