I have triggered this scene, but there still no request to dinner for mom. Weird.
Go right up to the point where you enter the Stepfamily house for dinner and then open the console. Enter the following:
Mom.TalkThemes["Requests"]["SFamDinner"].DinnerInvite = False
Mom.TalkThemes["Requests"]["SFamDinner"].Visible = False
StepFamilyTask.MothersConflict.SetState(QuestStates.Processing)
OldSis.TalkThemes["Requests"]["SFamDinner"].Count += 2
YoungSis.TalkThemes["Requests"]["SFamDinner"].Count += 2
Close the console and go to the dinner. Right after SMom gives you the cleveage shot, you should see Mary say they'd be happy to see your sisters also. Then you should have the conversation about the one complicated thing.
If you'd rather just skip to the end and avoid all the dinners, advance to 1800, go to the diner and type:
Mom.TalkThemes["Requests"]["SFamDinner"].Visible = True
StepFamilyTask.MothersConflict.SetState(QuestStates.Processing)
Go to the diner, talk to Mom and requst the invite to stepfamily dinner. She'll say no. Open the console and type:
Mom.TalkThemes["Requests"]["SFamDinner"].LockDate = GData.Date - 2
Then ask her again. That'll take you right to the "Let's do it" action