oh hey... thought I should toss in the standard disclaimer here, I know everyone knows... but I'm not a Dr., therapist, nor any professional qualified to instruct nor direct your actions in your life... These are strictly my opinions and not necessarily some guide that will solve anything...
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It is so tough... knowing we don not have to behave as victim, yet with such deeply ingrained behaviours, we don't see things as quickly, if at all, in ourselves...
It all begins with a choice... and a step... and then followed with course corrections...
You will agree that you are not looking to change your parents... you will need to begin with simple DB principles... NO EXPECTATIONS... keep your hope in check... we cannot control anyone but ourselves...
Even 20 years back when I began my own separation from the "madness" of my parents... I accepted that if a wedge got driven between us... well, that would be what it would be... I was the black sheep, so it was pretty easy to disappear for a year and then pop back in and say "hi"...
Get your boundaries in place, talk to your sister and let her know your boundaries and help her, support her, to create and hold her own boundaries...
Your mom (and your dad, too) will begin by getting a better understanding of the reality that their kids have grown up and will make their own choices... if they haven't before, they will begin to deal with empty nest...
They probably both feel they have the power stick and control others around them... the boundaries that you have will help cleave that long standing belief that YOU have... that they control you... they'll figure their part out, on their own...
Have your read "The Road Less Travelled" by M. Scott Peck? Also, you might want to look at a couple books by Richard Bach, "Jonathan Livingston Seagull" and "Illusions: The Adventures of a Reluctant Messiah". Awesome books for self reflection and discovery...