What is the word for what im doing to my cuz ? ( I know it aint blackmail)?

I don't think it is emotional blackmail. I just think you are being a smart a$$. Not really a big deal, of course he may not be able to take it like some people. But if your relationship means more to you than some joke, then you should quit harring him. You can only tease someone so much before they want to put an end to it. That holds true for family too.