I think there isn't much you can do. Your mom won't do anything and your sisters seem content to mooch off her.My mom's plan is to just even split 25% to all of us. So it would be a sizable amount to all of us.
What I don't want is to have my youngest sister and oldest sister just sit there at her house for the next 20 years or so while she 100% supports both of them. While also being cunts who just start demanding shit because they will somehow believe that they earned it or some shit by just being there. My middle sister and myself do well enough for ourselves and don't really have an interest in changing inheritance nor fighting over money and wills.
But my other two sisters don't really have a choice and their options become fewer by the day. At this point they'd be career waitresses who just haven't worked in years. I just don't want to see my mom exploited for the remainder of her life to support two mooches but I know she doesn't have the heart to kick them out. Compounded by the fact that I don't live there and can't directly help in it so much. I don't want to have a blood feud with my own sisters who, up until this point, I never had a problem with.
You could have a frank talk with your mom but that might turn out badly and start a feud with your sisters when they found out.