score:8
If you have decided not to go, then just cancel your interview. End of story. It's not going to reflect badly on you or your family, and it's certainly better than being denied a visa.
Also, I don't think there is anything at all suspicious about "my father is sick and my brother [you] is staying behind to take care of him". I'd almost say the opposite: since strong family ties are evidence that you're not going to immigrate, this should be in your brother's favor!