I'm going to make my family choose between me and my adopted cousin.
I can't believe you would do that. Why would you force them to choose like that?
She reminds me of my mother and it sickens me.
You shouldn't give your family ultimatums. You should be there for your adopted cousin and not force your family to choose between you two.
I can't help it. I don't like her, and I'm going to ruin my family if they keep her around.
Threatening to ruin your family if they don't abide by your ultimatum is extremely selfish and cruel to your family and to your cousin. Family members are meant to love each other. Making them choose between the two of you will just make things uncomfortable for everyone in the family.