Why is marrying your siblings illegal?
Its illegal because marrying someone so similar in genes, is dangerous to the health and well being of your hypothetical offspring.
What is it called when you marry a sibling?
Incest /ˈɪnsɛst/ is human sexual activity between family members or close relatives. This typically includes sexual activity between people in consanguinity (blood relations), and sometimes those related by affinity (marriage or stepfamily), adoption, clan, or lineage.
Can you marry your sibling in law?
Technically no. Your sister in law would be married to your brother. You can only be married to one person at a time. Your sister in law is related to you by marriage only, not through blood, so if she's no longer married, and you don't mind incurring the wrath of your brother, yes you can legally marry her.