Are you being serious or simply operating with a rather limited analytical framework? Sometimes I give people too much credit when it comes to critical thinking...
There's no incest, "Mom" refers to Meari's mom. Okaa-san (the word Sayoko used in the raws) is used for speaking to or about someone else's mom in Japanese
I hope Sayako is calling the picture “Mom” just because it’s Meari’s mom and not her own mom… right??!? this isn’t gonna turn incest is it