I was watching yesterday's episode, she calls the oldest lady in the restaurant unni. They are not related by marriage, because if they were the Chae siblings last name would not be Chae. She also likely did not marry the bio-dad of the Chae siblings, simply because the Chae siblings have her surname. This would be a lot easier to figure out independently if the kids surname was "Kang", as we can guess she either remarried or at least adopted them after their parents died, but it's "Chae" which implies their connection to the family is her. So something is definitely up with their family, they just haven't covered it yet. I guess we can see the way they picked up that third child as a hint to the family dynamic they got: Suji's bio mom picks up kids that don't have parents anymore and adopts them.
Yeah Uri's mom is obviously Suji's bio mom, they aren't hiding that at all in this show, I think the hidden mystery will be who are the Chae siblings real parents (if they even have the same set). We know there's a mystery dead person in the second lead's family that the mother doesn't want to talk about, maybe that's a lead that will involve one of the Chae siblings.