I have an MSR Twin Peaks - the immediate ancestor of the Sisters - that I added snow flaps to. I have used it on several ski tours; here it is at Dale Lake, Red Mountain Basin in the Sierra:
I like it - it sets up very solid, handles serious wind well. For two I dig the floor down about 8 inches for more room. The flaps are very nice for camping on spring snow - just pile on a few inches and it sets up hard and locks the thing in nicely. then I jack up the adjustable ski poles a little and get it drum tight. in dryer, colder mid-winter snow that might not work so well, but we don't get much of that dry snow in the Sierra.
Condensation is an issue but not bad as long as you get out early before it melts and starts to drip on you.
Would be very easy to add flaps to a betalight - it's all straight cuts and straight seams.