Hi PH. I think you're still too enmeshed with your sister. For instance when she texted you, you could have forwarded the text to your mother right away. You don't have to go along with your sister just because she wants you to cover. Nor do you have to act as policeman about who is telling lies and who isn't.
You still have expectations of her, when she's clearly shown she's not capable of looking after anyone but herself. No wonder your mother prefers you. If you don't expect her to behave like a mature responsible adult, you won't be disappointed.
As far as the abuse goes, everyone has different perceptions about their childhood. My sister thinks nobody understood her, whereas my other sisters and me always saw her as the favourite. Your sister is coping with what happened in her own way, making decisions as her right. I disagree with your therapist - you are not responsible for approaching her in any way, compassion or not.
I have 3 sisters, so I'm a bit of an expert on this. One is an alcoholic with a kind heart, one I'm close to, the other I can't be around too much because of some childhood issues. I have a certain amount of detachment about how they live their lives and regard it as none of my business. And I won't buy into dramas and disagreements.
Maybe you could work on being neutral with your sister? Detaching without being unkind. You don't have to pick up the phone in a meeting, or give her advice, or make up for your mother's attitudes. She's a big girl and she'll cope.
Just a final thought. Are you the big sister and do you still have the belief you have to guard and protect her?