Can a best friend's brother fall in love with his sister's best friend?
Yes and it is legal as the best friend is no relation to his sister.
If you are best friends with someone, maybe since childhood, and your parents have not met your best friends family you would want them to meet. Maybe the sister has been talking about her brother to them. It's just a friendship/family thing you do. Most likely have the best friend met your husband and his sisters parents. There is already a bond.
Your best friend's brother has no right to grope you against your will and it is time you learned to take care of yourself by learning the word 'no!' Tell him to back off and keep his hands to himself. Stay with your best friend while in her home or anywhere else and never be alone with her brother. If he persists then tell your best friend what her brother is up to.