i’ve only had sisters for a few months and never had mangrove, so some corroboration from a more experienced user might be nice, but here’s my take:
sisters sounds great as a sound source, but trying to make tuned counterpoint between a more “traditional” oscillator like mangrove and sisters may be frustrating. the center output is easy enough to tune via the frequency knob, but the other two (higher and lower) are much harder to nail down to a musical interval with the span knob. adding any FM makes everything totally crazy. i’d imagine quantizers could help get you the result you want, but that’s a lot of extra space and cash and maybe defeats the purpose. also i don’t think they track 1v/o?
for me, sisters is usually filter first, sound second. i mostly use the “sound source” side to add interesting harmonics to other stuff i’m filtering through it.