whoisthis you need to be kind to yourself. Spend whatever time you can with your Mum and dont fret about not being able to do what your sister does, being there and loving your Mum is the important thing. As for your DH, Son and the rest, let them fend for themselves, tell them why you feel as you do then leave them to it.
My OH was not very supportive when I lost first my Dad then my Mum but after having an almighty row with him it came out that he was lost not knowing what to do. It also brought back a lot from when he lost his Dad, its no excuse but you need to clear the air.
As others have said keep talking to us, we are always here for you.