Dating my cousins best friend- advice is needed

For the past month I’ve been dating my cousins best friend.

She gave him the green light that it was okay to pursue me.

I have known him for over 3 years and there has always been chemistry.

He takes me on dates and is wonderful. We have such a fun time and just laugh and smile.

However my cousin, who has a BF of her own, is jealous of the situation and often lashes out at me (when she is drunk). She will make fun of me and try and make me look bad. Keep in mind I’m 24 and she is 32. He is 29.

My guess is that she likes him more than a friend. However he does not view her like that. They have been best friends for over 6 years and nothing has happened.

We had a family dinner for her birthday and I made all attention on her and stayed in the background Bc her parents were meeting her new BF and I didn’t want any attention on him and I. (She recently announced to my whole family about me dating him. Didn’t give me the chance to tell my mother)

I did not sit next to him and I sat next to my aunt and talk to her the entire time. But he kept trying to ask me questions from across the table and engage in the conversation with us.

He very much likes me. How he looks, talks and his actions say it all.

She however kept grilling me the entire time and even her mother notice her attitude and my mom even said after the dinner that I completely did my best to stay away from her.

When we were leaving the restaurant, she didn’t say bye to me and walk straight to the car with her BF.

Mike gave me a huge hug and talk to me until my mother came out and proceeded to give her a huge hug and say what a pleasure it was to meet her. (This was the first time )

That night he texted me “hey I’m hitting the sack early but I wanted to say goodnight and that I had a good time. You looked beautiful.”

I’m going on a date with him tonight. I very much like him and all their friends adore me and say how wonderful the chemistry is. We are taking things very very slow. ( yes he met my mother but she wouldn’t of known if my cousin didn’t decide to tell everyone)

I just want us both to be happy and I don’t want her feeling this way. She is 32 ! She should want her best friend to be happy!

Tonight I need to word to him to keep us getting to know one another between us.. I don’t need to have this all going back to her.

How should I go about this situation ? It is 100% worth being with him. His best friend since preschool said that both you and him are happy and that is all that matters. You just need to stay out of her ear for awhile.

Why would she be for the relationship and then now that it’s happening freak out and be mean ?