So 'Sa' thinks that nani is Hindi for maternal grandmother but she also opines that papa and mummy are the Hindi translation for father and mother, respectively!
Weird, isn't it? Well actually, it's not. You might think that Sa's a bit soft in the head (and she might be for all you know) but this answer is, in an Indian urban context at least, spot on.
There is a particularly Indian mindset that we have, or maybe we're just misinformed, that language is something that's ossified in time and place. Which is absurd. The vocabulary of any language is an entity which, like the culture of a people, is in a constant state of flux. It changes; and one of the principle ways it does that is by borrowing from other languages.
For example, today the word 'jungle' is a pukka English one. Of course, the word has its roots firmly in India where it's still used. But that doesn't make 'jungle' any less of an English word. At some point in time, enough people calling themselves English speakers, started using the word 'jungle' as a synonym for 'forest' and voila! you have a new word. Why follow different standards for the reverse?
Anyone studying Hindi in school would, I'm sure, have noticed the rather vast difference between what's written and what we speak. It's like the recomended writers (Premchand is the only exception I can recall) are writing in this make believe language—one of the things that made me hate studying Hindi in school. Back in primary school (which we used to call junior school) a Hindi teacher of ours had expressly forbidden us from using the word paani for water. Paani, she said, was an Urdu word and we were studying Hindi (ironically, there's a fairly popular band by the name 'Jal' in a country that does claim Urdu as its National Langauge).
Now, without getting into the Hindi/Urdu thing, this is plain stupid. Most Hindi speakers would, almost exclusively, use the word paani for water. Just picture yourself saying, 'Ek glass jal dena'. Uh huh! Not happening. If just on the basis of origins (paani is of course, a Persian word) words were to to be kicked out of languages then English, for example, would lose more than 60% of its vocabulary (OED stats here).
Sometime in in the past, enough Hindi speakers decided to use the word paani and therefore today it is a Hindi word no matter what ma'am might say. And if today enough Hindi speakers use the word 'papa' to mean father (and I think they do, even going so far as to suffix a ji, as in papaji) then it becomes a Hindi word. This is how languages (and the mind of the great Sa for that matter), work. It's as democratic as you can get.