It only exists while the child remains a child. The parent-child relationship does exist, but is not recorded as such. That the two Sims are married is not recorded. Even when two Sims marry, what makes them part of the same family is that one moves in with the other. Two Sims are either members of the same family, or they aren't, and all Sims who live in the same house are considered part of the same family.
The family relationships in The Sims are very simple.
