I just caught up to this, and haven't read the Light Novels (or every chapter's comments), so I don't know if this has been raised/discussed before, but I started wondering. They say the homonculi lost their reason due to Solomon, their object of affection, disappearing. What if Bel has (accidentally) found the way to help them, by giving them a new object of affection? His daughter is devoted to him, and now the son has stabilized, after being shown affection, and possibly starting to develop affection, too.
If before, everybody saw them as just tools, then they likely never experienced anything close to affection from anybody other than Solomon (who may have shown affection, or may have just had enough control to demand affection). So it might be that if they are shown enough love, they can be stabilized again. (It would also mean it is important for his daughter to have others that she cares about enough, which seems to now be happening, or she could go berserk if he died.)