Is this wrong?? Getting pregnant behind his back

My 20 year old sister recently had a baby 4 months ago her boyfriend made it clear that he is not ready for another baby and probably won’t be for a very looooong time my mom has been telling her to have baby #2 already my sister wants too but her bf doesn’t she jokingly made a comment that she was gonna stop taking her birth control and purposely get pregnant this isn’t the first time she says this.. Idk why this bothers me I also have kids so no I’m not jealous but I hate that she thinks this way!