I am sorry to hear of your situation and I agree, you are almost 18 so it may be wise to wait on making any permanent decisions right now.
From what I have read:
A brother/sister share 1/2 the genes
1st cousins share 1/8 the genes
2nd cousins share 1/32 the genes
3rd cousins share 1/128 the genes
We only have 46 chromosomes. In other words, most 3rd cousins, as long as there was no previous incest, share no chromosomes and are not really related in any significant manner.
However that being said I would seek out the opinion of a genetic counselor to get expert advise seeing as this is causing you and your parent's problems.
You have been advised well so please know we care about you.
~~Kitt~~Moderator: Anxiety/Panic, Osteoarthritis, GERD/Heartburn and Heart/Cardiovascular Disease. www.healingwell.com"If you can't change the world, change your world"