I've looked into your mother's poisoning as you've requested. Turns out they never identified if the poisoned wine came from your house or not.
The night before your mother died she visited a friend, relative? We're having a hard time making the connection. His housekeeper says she had a glass of wine there, but went home for the evening before she left and doesn't know the nature of the visit. It wasn't a one time thing.
This man could have been targeted for the same reason that Mr. Fischer was a likely target-- supposed Deatheater connections that were cleared by the Aurors' investigation.
Enclosed is a photograph. Please let me know if you know who this man is in relation to your mother. His name is Robert Pucey.