It could originally mean vigil, well, literaly, "all-nighter".
παν-νυχίς (nyx/nykta for night)
Then it could make sense that it's used for mnymosino, because if you think about it, in older times when people died (especially if they died late in the evening & the burial was next day), the body was either brought to church or left at home & people took turns to read prayers over it (mainly the psalter) all night. So that was the first prayer service for the departed, which was a literal all-nighter.
But I'm only speculating there. It's almost completely disused in greek these days. I think I've only once seen the Paschal Service (before Christos Anesti) refered to as the Paschal
παννυχίδα in a greek church.
Are your aunts in the US or in Cyprus? If in the US, then it might be that they heard and adopted/adapted the russian word. If in Cyprus, then that's interesting

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