Welcome Harriet. This is a wonderful place to come for support and encouragement, and most of all, understanding! You sound like an amazingly giving and generous person, from the time that you dedicated to your mother, to your commitment to your granddaughter.
I agree with you that it's a good idea to draw some boundaries and reclaim some time for yourself. As I learned the hard way, you're no good to anyone if you spread yourself too thin and don't take care of yourself. It's a recipe for burnout. It may be tough for a little while, but your family will adapt.
On the days that you take care of your granddaughter, maybe your DIL could bring her over to your place? Would that be easier? She could even help you go through and organize some of your craft supplies.. I don't know about her, but my young daughter *loves* organizing things ; )
I also understand the healing power of organizing and decluttering..I hope you can find the time to tackle that so that you can spend more time on your creative endeavors. For me, my creative projects are a huge part of my healing journey.
Sending you positive thoughts and best wishes! We're here for you!
JT