Ask For Permission to Propose Interaction
Required Pack: The Sims 4 Base Game
Other Requirements: XML Injector
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So with this interaction, it was very tricky. The game does not have a father-in-law or mother-in-law relationship that Sims 3 did. You have to use the Discuss Future Together Interaction for this one to work, the buff lasts for 24 hours and as long as the Sims have that buff they can click on their boyfriend/girlfriend's parents and look under Friendly > Other Sims and use this interaction.
It will generate an in-text notification and this interaction should not be used for at least 3 Sim days again. This is in case the Sim decides to not marry in this relationship and allowing it to be used again.