how do i tell my best friend about my relationship?

So in november i started talking to this guy and he makes me a better version of myself. i’ve never been happier and he asked me to be his girlfriend in december. he lives in florida and i live in new jersey so we don’t get to spend much time together but i said yes anyway. i can’t imagine my life without him. but there’s an issue... he is my best friends cousin. he’s the only family member that she actually likes. i’ve tried telling her multiple times and every time she reacts differently. sometimes she says “i’m okay with it. i just need some time to adjust to this.” and other days she says “if you guys were to ever get together i’d kill both of you” i’m so confused and i don’t want to lose either of them.