Your life, bro... but you did ask, so here goes:
First, I would try to reconcile with my mom. Life is too short and I am not a fan of living with regrets. Job or no job, I would try to open communication with her if nothing else.
Next, I would talk to the cousin and let him know I was interviewing for the job. I would tell him if I got it that my plan was to work professionally and that I was not planning on becoming best buddies - but I did hope for a civil work environment.
If he wants none of it, I might pass on the job. Stress is a killer, and I'm sure he could make life pretty miserable, even if "he isn't supposed to" by company policy.
Either way, I am hoping for whatever is best for you and your home.
