Yevamot 30: Adding a Non-Sister to the Mix

5 mishnayot, with templates of 3 brothers, two of whom marry two sisters, and the third brother marries an unrelated woman. The cases play out the different variables and their implications for yibum and chalitzah for the remaining family members. Plus, a mishnaic comment on the 15 co-wives from the first mishnah of Yevamot.