I don't know if there is chemical addiction, mental illness or something in your family, but my FD's family is the poster child for dysfunction. Perhaps AlAnon would be helpful because it helps to explain to us the role we play in the dysfunctional family and the codependency.
Someone explained the triangle of dependency to me and I am so grateful. Wow, it really was a lightbulb to me. I don't know if this is true of your family, but possibly you may see yourself in this triangle.
Usually in a dysfunctional family, the parties of dysfunctional move around but generally there are three parts,
1) rescuer (they need to be needed) - your mom
2) victim - your sister
3) persecutor (that's you according to mom and sister) - not in reality but it supports your sister's illness and mom's needed to be needed.
I thought was said well here - if you would like to read more about the triangle.
http://www.my-counseling-site.com/co...ounseling.html
When you remove yourself from the triangle, they will need to find another persecutor or will continue to blame you in your absence.
In codependency, we all have a role until we have appropriate healthy boundaries.
I know because I moved between being the rescuer for my FD (due my need to be needed) and the persecutor (according to FD and her biomom or at times between the biomom and the biogreat-grandmother).
I have removed myself from the triangle and things are much better. I am still around FD's biofamily but I don't get into the drama. I listen and nod my head but don't try to help and solve all their problems (I couldn't anyway - it is a never ending cycle of need and drama). And I am not blamed the way I used to be by biofamily or FD.
Since I don't play a role anymore, I am much healthier and FD is seeing the family's drama a lot more and doesn't get involved like she did before. She's also learned the the codependency she had with biomom was a toxic kind of love and not a healthy love, because it was based on secrets and lies against the third part of the triangle (sometimes me, sometimes biogreatgrandmother).
Things are a lot better between FD, me and biofamily. (I'm her legal guardian so we have an ongoing biofamily relationship). Still not perfect but definately better.
(Also, at the last holiday dinner, I discovered that a glass of wine before we go over there helps a lot too).