I am dating my sister's boyfriend's brother and everyone is telling me that if they were to get married, my boyfriend and I would be related. But I'm almost positive that's not true?

He would be your brother-in-law.

There's no blood relation, so it's not illegal and won't cause weird children. Wikipedia says, "A brother-in-law (plural brothers-in-law) is the brother of one's spouse, the husband of one's sister, or the husband of one's spouse's sister." In the Bible (Old Testament), if a woman was widowed, her husband's unmarried brother was to marry her in order to keep her in the family and for protection.

Wondergirl is absolutely right. There is no blood relationship at all, so you don't need to worry about being involved with a "brother-in-law's" brother. There's nothing illegal or immoral about it.