This is great. What a thankless position this is.
I am, luckily, not a SM anymore, although I have been. My ex's ex used to refer to me as 'his fucking girlfriend.' which was lovely.
I hate to break this to you, but it doesn't ever seem to end. My DMum married my DSDad 20 years ago. They are in their 70s - and now the arguments with family have been about ownership of their parents!
My step dad's mum moved in with them, and my DM cared for her for 3 years until she died.
When she died, she left £10k to each of her 3 grandchildren and gave the rest to my DMum and DSDad.
My DM and DSD very generously decided that they wanted to pass on their money to their children (3 on each side) now - when we need it most - rather than wait until we inherit, so split the rest of their inheritance 6 ways.
The fallout brought out a really nasty side in the other side of the family. The money was DSGrandmother's not my usurping, interfering DM's. She was ok to provide care, but was not, in any circumstance, allowed to benefit from her husband's inheritance and certainly, us grabby step siblings shouldn't see a penny.
It was horrible. Mum ended up giving them the lot to keep the peace, even though they had already had their own inheritance.
It's all silly. My own DGM is still alive and I'm not remotely fussed about what my DM does with any money from her. I hope she takes my DSDad on a lovely, extended holiday - and if they want to dole it out to my step-siblings then that's their choice!
So step-parents beware... there are even more issues to look forward to!