A man is not allowed to walk between two women. Does this apply where one of them is his mother (or sister)?
The source is Pesochim 111a and Horayos 13b where it says that it's koshoh leshikchoh (harms one's memory). Regarding relatives, Sefer Shemiras Haguf Vehanefesh page 333 brings from one sefer that it is mutar (allowed) (because the reason it is koshoh leshikchoh (harms one's memory) is because one diverts his attention from Torah to an Ervah, (so it doesn't apply to relatives).
However, all the other seforim don't make this distinction, so it appears that they disagree.
"A man is not allowed to walk between two women."
This whole notion appears in an aggadic, somewhat spooky-things-that-go-bump-in-the-night portion of the Talmud. It does not appear in Rambam's code of law, nor Shulchan Aruch.
Some Jews (like the one asking this question, and the latter-day compendia cited in other answers here) still feel this is binding and are worried about it.
Others feel that the Talmud may have been conveying a deeper message or the like in passages like these, however, as for practical action, if you're not spooked, it won't spook you, so no need to worry about it.
So take that as you will.
The Shulchan Aruch Harav says that the reason one is prohibited (and it says that the prohibition is in force nowadays) is because of the "evil spirit" there (and not because of Tznius).
Therefore, at first glace, there should be no difference whether the two women are related to him or not.