Mods / Traits: Custom Scenario: Sibling Rivalry by DaleRune – Mod The Sims 4.
This one is basically a merger between Living with Enemies and Too Many Toddlers: it requires two YAs+ and two toddlers (preferably the set-up should be that the adults are siblings and the toddlers are cousins, but I have yet to find a way to program it so the Sims must be related to each other, as opposed to must not). There’s three listed endings to be found, explained here for ease of in-game legibility.