My cousin hates me cause im dating her ex, help?!?

my cousin dumped her ex boyfriend like 7 months ago... me and him got to talking a few weeks ago, and now we are dating, and her mom and herself cannot stand the fact that i am dating him. they said i shouldve "asked" first... its not like im borrowing a toy or somthing right? idk i just think its rediculous how immature they are acting. and my cousins dad is on my side, he thinks its non of their business. and she has had 2 boyfriends since him. they all say that he and i will not last but them not wanting me to do it makes me want to even more. She and i have never been close, What do you think?