(Exodus 21:9-11) "And if he designates her for his son": for his son he may designate her, but not for his brother. For one might have reasoned: if the son, who does not stand in the father's place for levirate marriage, may designate her, then the brother, who does come in his place for levirate marriage, surely by reason should be able to designate her. No: if you say so of the son, who enters in the father's place for inheritance, will you say so of the brother, who does not enter in his place for inheritance? Therefore he may not designate her. Scripture teaches, saying "for his son he shall designate her": for his son there is designation, but there is no designation for his brother.
A Son May Marry the Maidservant but Not a Brother
Yalkut Shimoni on Torah 320:6
(שמות כא ט-יא) וְאִם לִבְנוֹ יִיעָדֶנָּה, לִבְנוֹ יְיַעֵד וְלֹא לְאָחִיו. שֶׁהָיָה בְּדִין, וּמַה אִם הַבֵּן, שֶׁאֵין קָם תַּחְתָּיו לְיִבּוּם, הוּא יְיַעֵד, אָחִיו שֶׁהוּא בָּא תַחְתָּיו לְיִבּוּם דִּין הוּא שֶׁמְיַעֵד. לֹא, אִם אָמַרְתָּ הַבֵּן, שֶׁהוּא נִכְנָס תַּחְתָּיו לַאֲחֻזָּה, תֹּאמַר בְּאָחִיו, שֶׁאֵינוֹ נִכְנָס תַּחְתָּיו לַאֲחֻזָּה, לְפִיכָךְ לֹא יְיַעֵד. תַּלְמוּד לוֹמַר לִבְנוֹ יִיעָדֶנָּה, לִבְנוֹ יִעוּד וְאֵין יִעוּד לְאָחִיו.