Simplify the following text for a Grade 4 native English-speaking person.
Original: A sister-in-law ( plural sisters-in-law ) is the sister of one 's spouse , the wife of one 's brother , or sometimes the wife of one 's spouse 's brother .
Simplified: Someone 's sister-in-law is the wife of his or her brother . Someone 's sister-in-law is also his or her spouse 's sister .