My brother-in-law is 73 and his wife of over 40 years is 71. She is currently getting less than half of her husbands social security based on her social security earnings. Can she still apply for the spousal benefit and either get a) 50% of her husbands social security or b) the difference between her social security and 50% of her husbands social security? If not, then why not?
She can file, but I don't know if she'll be eligible for additional benefits. Your brother-in-law's wife's excess spousal benefit would be calculated by subtracting her full retirement age benefit rate (i.e. the amount that she would be receiving now if she started drawing at full retirement age) from 50% of her husband's full retirement age rate. So, if she started drawing her benefits prior to full retirement age, she may not be eligible for any additional benefits.
For example, say Mary has a full retirement age benefit rate of $800, but decides to file at age 62 and take a reduced rate of $600. Her husband Bill has a full retirement age rate of $1500, but files at age 70 and receives an increased rate of $1980. Even though Bill's monthly rate is more than 3 times as much as Mary's rate in this example, Mary would not be eligible for any spousal benefits on Bill's record, since her own full retirement age rate ($800) exceeds 50% of Bill's full retirement age rate ($750).