First, a couple detailed things: she confirmed that although 'bhaiya' is the more familiar/affectionate form of 'bhai', the '-ya' ending was not generalizable in the way that, say, '-ji' is generalizable. She was not able to think of an equivalent way of making other relationship terms more familiar/affectionate. That's what you all had told me, but I thought you might be interested to have it confirmed.
On the other hand, she said that 'guru' was a term which could be applied to a woman just as easily as to a man. Her impression was that non-Hindi-speakers thought of it as having a male meaning because the most famous gurus were generally men, but stated that any woman who was a teacher or mentor, with whom one had a relationship of respect (not friendship) could perfectly well be called 'guru' or 'guruji.'
More broadly, and to me both interestingly and a little dismayingly, she looked at http://www.shabdkosh.com/ when I showed it to her. We looked up 'sister' as an example. Her opinion was that the site was slanted toward very "proper"/somewhat old-fashioned Hindi. I pointed to the term given for 'little sister' in particular and asked if that would realistically be used. She said possibly in a village, where people were very traditional, but definitely not in a city. We didn't review the site and other words enough to see how consistently this slant was represented.
It's made me feel a little cautious about using the dictionary, though, so if anyone knows of a more current/idiomatic dictionary (online or in print) which would give more information on the nuances of different words with similar or identical definitions, I'd be grateful to hear about it.
I don't want to abuse her patience, but she actually seemed delighted by my interest and not dismayed by my areas of ignorance, so if anyone has questions they haven't been able to get satisfactory answers to - the way I did, for instance, about a female mentor - that you would like me to relay to her, I would be willing to see about it. Obviously, she's just one person, and part of a specific Indian diaspora community. But she's very well educated, very fluent and idiomatic in US English as well as, apparently, Hindi (and I have no idea how many other languages), and has offered herself to me as a resource.
Hope at least some of this was of interest to someone.