כִּ֚י אִם־לִשְׁאֵר֔וֹ הַקָּרֹ֖ב אֵלָ֑יו לְאִמּ֣וֹ וּלְאָבִ֔יו וְלִבְנ֥וֹ וּלְבִתּ֖וֹ וּלְאָחִֽיו׃
si ce n’est pour ses parents les plus proches: pour sa mère ou son père, pour son fils ou sa fille, ou pour son frère;
Tosafot
"In what did He immerse?" -- the heretic did not ask, "how was He allowed to become impure [by burying Moshe]?". This is because the Jews are called children of God [and kohanim are allowed to become impure for their deceased children].