Hi, here comes another WIP!
First, all twins now get the status of older/younger in relation to the chosen one. You can track the status in a sim profile.
This status is assigned automatically for both newly born and already born children. For the first, the order is determined at birth and does not change from the order in which you will grow up your sims during the game. For the latter, the order is determined by age progression. But you can change the order with the help of cheats on the sim.
Second, toddlers, children and teens may argue with each other about who was born older.

You can also ask the parent who was born first. Depending on whether it's the older twin or not, the sim will be happy or angry at the unfair coincidence x).

Third, I made a custom relationship scale for twins! Now there is satisfaction between each pair of twins in their relationship. It changes not only from normal social interactions, but also from twin-specific ones - and much more strongly.
Roughly speaking, just to insult is one thing, but to say that only one of the twins got all the brains, and the other was born for nothing - WELL, another. If the scale drops below zero. then the icon and signature will change to “Don't like having the twin”.
Well, I also did a little work on the icons :)
Fourth, a “play mind-reading” action is available for children. Depending on the combination of the friendship scale and the twin's satisfaction scale, children will receive one of four moodlets. With some probability, they may also learn something about the twin's personality: character traits or preferences!

That's all for now but I have a few more ideas!