A very difficult time for you, and lots of very positive helpful suggestions. I am too far away, in London, to help but if I was closer I would come and help for a day, so much easier if one doesn't have the emotional involvement.
I needed to totally declutter and my goddaughter who was on holiday from university came with her friend for three nights over the New Year. I paid them for 8 hours work a day, fed and housed them and took them into London for New Years Eve. In return they got rid of masses of stuff, they were ruthless, it was wonderful! Stuff from my family, old furniture and books I had for 40 years or more, even from my grandmother, clothes that the charity shop would use but which my lovely goddaughter said that I want allowed to keep (she was right). I now have memories and photos, not out of date furniture and ornaments. Have not missed anything that went, but feel SO much better!
I then read the Marie Kon book on learning to love tidiness and decluttering, Japanese, minimalist, fantastic - and am doing some more!
Don't be too hard on your daughter - she will be with you for several days it sounds, and she has to think of her husband and children. Please don't be jealous of her time with her SIL - family dynamics are always a mystery! Hope it goes well, am sure it will.