The Trinity siblings are most likely half-siblings, as proven by their blood types - Johann is type A, Michael is type O, and Nena is type B. Due to both A and B being dominant, the only way they could all share both parents is if one parent was type A and the other was type B but both were carriers for type O. It’s canon that Ribbons Almark is their father (#1 asshole dad of the year!) but nothing is known about their mother/s.
Johann’s large (physical - they’re, physically, 26, 20, and 17 respectively. Mentally they’re all 7. Confusing, I know.) age gap also indicates that he was created long before Michael and Nena, with him being six years older than Michael and nine older than Nena, whereas the younger two are only three years apart. This could point to him at least having a different mother and something making her later “unavailable”...
There is another factor that weighs in favor of this theory but that’s something I shan’t touch with a 39 and a half foot pole.