Okay, I need to get this out there.
I do not believe Sarah is Ren's mom, but his sister.
Hear me out:
1: His mom's name is Juli; she wouldn't have a reason to not go by Juli
2: In the scene where we get a peak at Juli, she's not wearing glasses. While it's possible for her to wear contacts, why wouldn't she wear them now?
3: Sarah has a large appetite, much like Koo. This would make a lot more sense if she was, in fact, Koo's daughter.
4: When Koo referenced Ren as his son, he never said "my only child", he said "my only precious son", and in fact, and any reference to them having kids, it was only sons [Kyoko now being the other son]
5: I don't believe Juli would have risked going to Japan, especially when Koo would stop her from going all the time before, and especially now that she has the video.
6: Koo talked about how Juli wanted a happy family, and often times people who want to have a family will have more than one child. It would make sense for them to have another kid in this case
7: Ren's surprise would be even more warranted, because why would his sister come to Japan?