How Old Is Eiko?

If I had to guess, I would say Eiko was 90 years old. I was, however, proved woefully inaccurate.
Eiko is one of my students in my weekday lunchtime class. She makes up for her lack of English with a face cracking smile. Every afternoon she will shyly shuffle into the room and will painstakingly gather her limited vocabulary to comment on the weather, and will remain mostly silent for the remainder of the lesson. As much as I try to get her to join in, her self-conscious lack of understanding of the language keeps her mute.
I make it my business to let every student participate in my lessons, so as an introductory warm-up I ask each student to comment on their weekend activities. The other more able students in the class can do this with relative ease. Eiko, however, stares at me as if I have asked her to describe Einstein's theory of relativity. I understand why she might be reticent, so I carefully craft my questions which only requires a 'yes' or 'no' answer.
But one day, she astounded me by constructing a detailed answer to my question.
"I met my mother-in-law and we went shopping and played tennis," she chirpily said.
The initial rapture I felt that she was beginning to speak English gave way to troubled curiosity at the viability of her statement.
"Your mother-in-law?" I asked Eiko, whilst covertly counting the numerous wrinkles on her weather beaten face.
"Yes," confirmed Eiko.
Without seeming rude, I aborted scrutinizing her features and composed myself for more investigating. I had a gut-instinct that I was about to discover the oldest living mother-in-law in existence and I wanted to make sure Eiko had got her facts right before I notified the relevant science magazines.
"Are you sure you mean your mother-in-law? Don't you mean your daughter, or cousin?"
"No, no, my mother-in-law. We buy gift for my daughter's birthday. Next week is daughter's birthday."
I searched my brain for the Polite Questions Department and happened across this belter.
"And how old is your daughter?" I asked.
"She is 70."
I looked at Eiko in disbelief, then at the class, who were busy trying to not to laugh.
My brain passed me another question from the Polite Question Department.
"So how old is your mother-in-law?"
"50"
I mentally drew Eiko's baffling family tree and came to the conclusion that her daughter was older than her. I broke out in a sweat as I tried to figure out this complex puzzle until I realized my palpitations and ruminating silence cast an unsettling atmosphere in the room. But there was another aspect of her story which puzzled me.
"Tennis as well? Can you play?" I asked, thinking that she would surely snap in two if she tried to pile-drive an ace-serve.
"Yes. I'm good player. But I lost," she said, and made a sad face.
I was about to say I wasn't surprised seeing as your more nimbler mother-in-law is 20 years younger than you, but I refrained.
There was one piece of the puzzle missing that would have explained everything, but the Polite Question Department must have deemed the question, "So, how old are you Eiko?" beyond the pale.
This little mystery will surely remain unsolved.