Well just send a card saying how much you fondly remember her Mother but at the moment you are very busy and have no time at all for meeting up. Don't say "I'll be in touch when I'm less busy" or anything which gives the idea of that...just reiterate how you remember her Mum and that you wish her well.
An elderly lady who was a friend of older generations of my family (all dead) died in February. I found her interfering and increasingly avoided contact with her because she would ask awkward questions and be critical. My whole relationship with her was full of incidents where she interfered and now she's gone I feel I want to draw a line under the experience.
I exchanged a few polite letters with her daughter following her mother's death. By chance I moved house recently and now live only in the next town to her. She has suggested I meet up with her and her h as we live so near. I don't want to. I've replied putting it off for now, saying I'm too busy at the moment. How do I avoid meeting up?