20 CFR §725.223

1. (a)A parent, sister, or brother is entitled to benefits beginning with the month all the conditions of entitlement described in § 725.222 are met.
2. (b)The last month for which such parent is entitled to benefits is the month in which the parent dies.
3. (c)The last month for which such brother or sister is entitled to benefits is the month before the month in which any of the following events first occurs:
   1. (1)The individual dies;
   2. (2)
      1. (i)The individual marries or remarries; or
      2. (ii)If already married, the individual received support in any amount from his or her spouse;
   3. (3)The individual attains age 18; and
      1. (i)Is not a student (as defined in § 725.209(b)) during any part of the month in which the individual attains age 18; and
      2. (ii)Is not under a disability (as defined in § 725.209(a)(2)(ii)) at that time;
   4. (4)If the individual's entitlement beyond age 18 is based on his or her status as a student, the earlier of:
      1. (i)The first month during no part of which the individual is a student; or
      2. (ii)The month in which the individual attains age 23 and is not under a disability (as defined in § 725.209(a)(2)(ii)) at that time;
   5. (5)If the individual's entitlement beyond age 18 is based on disability, the first month in no part of which such individual is under a disability.