Re: MARRYING YOUR COUSIN...
^I'm sure parents are aware that type of feeling so more often than not they would not think of marrying off to someone who you have grown up with and have that brother/sister type of relationship, and you've forgotten about distant cousins, and medical problems only arise if this goes on generation after generation. And that "extremely sick" is sure to offend guppies who are married to their cousins.
Personally I would marry a distant cousin, not anyone that I've known for awhile.