To affiliate with a family through marriage; to marry into a family; (for parents of a married couple) to be related to each other through the marriage of their children (si-).
It is improper for a person to mention his mother or father-in-law by name.
It is hard for a poor person to marry into a rich person's family.
Lay ko kapamikit ni paglūng Pa' Juhil.
You happened to marry into Uncle Juhil's family.