During the end of Morrison's Run Nova told Martha she could call her "Ernst" and this lead everyone to believe that Nova had been Ernst the entire time, but that doesn't make sense.
If the X-Men knew she was Ernst wouldn't they have freaked out when Ernst joined Xorn/Magneto's Brotherhood? Nova+Magneto wouldn't have been good at all
And then after Morrison when the Mansion was destroyed Beast and Cyclops freaked out that Nova wasn't in the basement anymore.....but if she had been Ernst she never would've even been in the basement at all
So i think either:
#1 Nova at the end of "here comes tomorrow" was Nova inside of Ernst the way she tried to get into Hisako, or;
#2 Martha was semi-altzheimerish and just thought Nova was ernst b/c they're both old ladies
(you have to remember that Nova's body was destroyed by the nanites she injected into herself. The mansion was destroyed while ernst was part of Magneto's team (so nova being in the basement AND in the Brotherhood would've been impossible)
I think Ernst was just meant to be a little mutant girl who's power was physically to look old