The 31-year-old has been dating Segel for around six months and her six-year-old Matilda, with the late Heath Ledger, is keen for Segel to become a part of the family, reported Contactmusic.
"You should hear Matilda talk - she wants to be a flower girl, and she tells her mom she wants a little sister. She knows that won't happen if Momma doesn't marry Jason - so she's pushing for it," a source said. Williams is said to be smitten with Segel and is considering tying the knot before the end of the year.
"Jason lives to make Michelle laugh, and that's pretty hard for a girl to resist. Plus, he's shown he'd make a great daddy. Bet on a marriage by the end of the year," the source added.