I can't relate to seeing the apparition you saw, bb, but I have a thought or two why he - or she - might have been there. The location, the house, is probably of no significance. What you've indicated about your mother's family most likely IS.
Mikey explains many events and occurrences as down to 'energy' and the energy of an unsettled/unhappy incarnate is something a discarnate may be attracted to. The potential reasons range from a concerned 'dead' family member wanting to help thru to a mischief-maker using that energy to tease, upset or frighten someone. Or it might be used just to enable a discarnate to draw near to the earth dimension without any particular intention, kinda out of curiosity about what would be experienced.
Alternatively their reason might be a burning desire to again get involved with this earth and its inhabitants.
Quite possibly, mac. Insofar as I got any "feeling" or impression from the being or whatever it was, though, I don't think it was a dead loved one or family member. I honestly don't think it was ever human. As I mentioned, although I was terrified, I didn't feel malevolence from it; I didn't really feel benevolence either. It is truly a mystery to me.