What is it called when a brother and a sister date?

In English, the preferred term appears to be double in-law marriage. The two couples become double in-laws or simply double couples. Sisters and brothers married each others brothers and sisters, becoming double in-laws.

Can a boy marry his mother sister daughter?

A girl can marry her grandmothers (Mothers Mother) sisters sons daughter. But a boy cant marry his grandmothers (Mothers Mother) sisters daughters daughter.

Can I marry my fathers sisters husbands daughter?

Yes assuming you are Hindu, you will be governed by Hindu Marriage Act and according to this Act, you are not related to each other within the degrees of prohibited relationships, hence you can marry her. For example a Daughter can not marry her father and grandfather.