marrying your relative

Because you share the same genes. There is a stronger chance for recessive diseases to manifest in your children, like for example, hemophilia.

Umm.. you are talking about incest? I don't really know the "official" reasons for it but an unofficial one might be something I heard.. that the offsprings from such unions typically end up with weird diseases or become stupid... not sure if there's any truth in it though.

For some reason society says that is wrong and I see no reason why. As long as you don't have children, I see nothing wrong with sharing your lives together. Often, siblings are best friends. Children born of siblings often do not develop normally mentally and physically, but I suppose you could adopt.

i heard from the chinese culture that it's a practice for some to choose a distant relative to marry. "keeping it in the family", per se. "it" would mean, preserving the culture, maintaining the family wealth within the family, etc. but i haven't heard of siblings marrying. but generally, we can't because: biblically, it's called incest. a sin. genetically, there's a high probability that a serious impairment will occur for the children that will be born out of this union. for example, if diabetes run in the family... and same family members (both carriers of the same genes) produce a child, you run the risk of the child having a diabetes too. almost a 100%. why would you want to marry your own brother or sister anyway, when there's about 6 billion other people in the planet?