In my head, Su accidentally got pregnant with the twins almost right after giving birth to Opal. The pregnancy was really difficult on her and Katara and Kya had to be the ones to take care of her through it.
When Lin learnt of this, she almost reached out, but ended up chickening out at the last second. This woman would rather fling herself into a volcano than expose her emotional vulnerabily.
During this time, Aiwei inserted himself into the family and helped Su with managing the city and stuff. He even occasionally helped babysit the kids. This is why Su was so attached to him. In my brain I also like to think that she named the older twin, Wei after Aiwei. They already had a Junior in the house so they just shortened the name i guess.
After a very difficult birth, Katara advised Su not to try for anymore children. This is also one of the reasons Su ended up adopting Kuvira when the opportunity presented itself.
Being so close in age, Opal and the twins are often treated as a set of triplets rather than being just short of a year apart. This also means that, despite being younger, the twins feel very protective of their sister, especially back when she was a nonbender. I can also see Su being, in turn, super protective of the twins. They're her baby boys and she always wants them close to her. A little bit of codependency going on there I think.
