For a mature lady, the very fact that she is given in her mouth and cum in the same place is like a balm to her body. She feels that she hasn't lost her attractiveness and competes with her younger girlfriends on an equal footing. And the attention of the males tickles her vagina so much.
It seems after the guy fucked the stupid sister - her mind came back to her. That's what a swallow of fresh cum does to bitches!