She wants to, but doesn't have time to react.

Lisa was already imagining the taste and feeling of her big brother's cock in her mouth, but she was so surprised by Marge's sudden reaction that she didn't have time to do anything before her head was pushed down on Bart's dick. Marge didn't care a bit when she felt some resistance from Lisa and kept pushing her head further towrd Bart's crotch. As soon as he felt his sister's mouth close around his shaft Bart quickly took hold of her head too; so if Marge had stoped pushing, Bart would have kept forcing his cock in Lisa's throat. The poor girl began gagging hard as the intruding member was **** down her throat, but when his whole length was lodged in Lisa's gagging got worst and she began **** violently.