I'm very sorry about your father, but I'm just going to play devil's advocate for a minute. Hopefully, you've never lost a child and so you can't imagine how she felt knowing she was outliving her son. Maybe she was running away from that, KWIM? A cross between fear and denial.
Unless she and your father had an awful relationship and you think it was out of malice, I don't think you should hold it against her. It's been a long time, and if she died tomorrow I bet you'd have some pangs of regret for holding a grudge.
Having said all that, I still think you should go, just ignore the request for $50.