There's no such episode.
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Meg has her 17th birthday in the episode 'Peter's Two Dads' (UK Season 6/US Season 5) Meg turns 18 in the episode "Meg and Quagmire" (US Season 10)
There's no such episode where Lois does it in front of both Meg & Peter.She is however, caught kissing Meg's boyfriend in Go Stewie Go
Season Five|Episode Eight Barely Legal
Season 4|Episode 23 Deep Throats
He never really beats her up exactly but he hits her with a bat in "Griffin Family History". He punches her when he's on steroids in "The Father The Son and the Holy Fonz"