Hello. Your family sounds lovely.
I would match this vile and vindictive behaviour with the same energy. I would send mummy dearest a letter and send copies to your darling siblings. The letter would say something like this...
Dear.....
Regarding unwanted tat clearance costs when the old bat pops her clogs and proposed associated costs, or any proposed lawsuits.
Hahahahahahaha....( fill almost the whole page with this apart from the last couple of lines).
When the old codger finally kicks the bucket, you can stick any unwanted crap removal costs where the sun don't shine.
Give my regards to Beelzebub on your descent to the Earth's core, you owd bat.
For good measure I'd then consult with a solicitor to see if you can go complete no contact with all three of them.
I did have another thought about, once she's passed, clearing the house but just leaving the unwanted crap in the back garden, but honestly, on reflection, it seems like too much time and energy to spend on it.
You don't owe these grifters anything.
Good luck!