That's interesting. I have noticed my mother sniffs at the end of a sentence sometimes. Thinking though about what she has just said before she sniffs, she seems to do it when she's said something that makes her feel superior: for example, she will say 'we don't watch much television, we listen to music instead", and then gives a little sniff. Does the sniff signal the end of the sentence - or is her sniff a mix of her personal micro-expression of disgust mixed with a slight feeling of superiority: she doesn't really like television, and sees watching television as inferior compared to music listening.
I'll be watching her more closely next time I see her, to pick up more clues