Stick with Nex or Head Back to Micro Four Thirds

You don't really specify what's wrong with the Sigma 19mm for your needs? Anyhow, unless you want to go legacy/manual glass, I would bet that the upcoming SEL3518 will solve all your problems - the perfect normal length on the NEX APS-C sensor, fast for low light, optical steady shot stabilisation, and probably very sharp with nice bokeh (if the SEL5018 and alpha sam 35/1.8 are anything to go by). Also, why not up your ISO? Hard to believe that any M43 is better than 5N in low light. Try anti-motion blur. Dialed-down DIY bounce flash can look very natural and available-light-ish, like this with old NEX-5 and Canon FDn 28/2.8:

Alan