Sans and monospace font match for STIX Two
i've checked with the font developers charged with "cleaning up" the stix 2 fonts.
the basic text font is modeled on times roman, as noted.
the sans and monospace characters in unicode plane 1 are the outlines from stix 1. they are both slated for cleanup and some redesign (but as far as i know that is not yet scheduled). the developer says this:
I don't know if you could say that there is a direct model for any of the sans, it is simply intended to work decently with the rest of the font stylistically. The math monospaced glyphs are also the outlines from 1.x, [...] but as with most other math monospaced are a modified Courier.
regarding selecting compatible fonts for sans serif and monospace text, again from the developer:
I think if one had to choose a matching Sans, there are a number of candidates, but yes, Helvetica and Courier are as good as any, if we consider availability issues.
there is plenty of advice in other questions on this site regarding adjustments in relative size (e.g., normalizing x-height) to these fonts for the sake of compatibility of appearance.