Yeah, theres many possibilities on it depending on choices, i dont even think its smart to touch this matter because it changes too much stuff, example:
Imagine if Miru is already pregnant...then would be crazy all the pornstar stuff, open relationship, NTR...everyone would be filling Miru while Ayua is there, she would be a chimera

If shes not pregnant theres possibilities like,
Ayua and her sisters are someone's else offspring, but it was after MC and Miru broke up.
Ayua is William's, but he and Miru broke up and her sisters are from someone else.
Ayua is someone's else, MC ended up with Miru and knows it. (open relationship + MC cuck), her sisters are MC's daughters.
Ayua is someone's else, MC ended up with Miru and doesnt know it (NTR), her sisters are MC daughters.
Ayua and her sisters are William's.
And with the open relationship stuff even her sisters could be from different father or whatever.
If William ends up with Miru but is also with Katie or Leia, or both and this goes into SG's plot, players will want a harem ending in SG too.
Too many different stuff for the crybabies to complain.
But if its only for some dialogue flavor or minor stuff it can work imo.