The hypothetical woman who marries seven brothers is built upon a real biblical institution rather than an invented marital custom. The Sadducees appeal to the law associated with levirate marriage in order to challenge belief in resurrection.
Understanding that law makes both the logic and the artificial escalation of their question clearer.
Deuteronomy 25 describes a situation in which brothers live together and one dies without a son. The surviving brother is to take the widow so that offspring may continue the deceased brother's name and inheritance line. The practice served family continuity within Israel's kinship and land-based society.
The Sadducees extend the case repeatedly: each brother dies childless, leaving the same woman successively married to all seven. Their final question assumes that resurrection would simply reproduce earthly marital arrangements and thereby create an absurd conflict.
The legal background shows that the Sadducees are not primarily asking Jesus for pastoral advice about an unusual family. They are constructing a Torah-based argument against resurrection.
Jesus answers by challenging the premise that resurrection life is merely present social life restarted. He then argues for resurrection from the Torah itself, meeting his questioners on scriptural ground they recognize.