I have a few suggestions. These are for your benefit but also for theirs.

1.Learn about emotional blackmail and how to deal with it. Your sister is doing this when she gives emotional pain to anyone who doesn't do what she wants.

2. Limit the amount of time that you are willing to listen to them. Thirty or forty-five minutes maximum per person per day of untrammeled complaining is plenty, overly generous even, under the circumstances. Ignore the inevitable ensuing attacks that you don't care, etc. Just ignore them! You have a life, or should, and you are doing them no favors by letting them do whatever they want to you.

3. Do not allow your sister to interrupt you when you are busy. Agree on a time later on, unless she has used up her time for that day. Again: ignore her when she says that you lack empathy! Seriously, you have so much concern for her that what you give her is excessive and disrupting of your own life.

4. Insist kindly but firmly that your mother address complaints about your sister to your sister. Don't listen to such complaints.

5. Rarely if ever offer your sister any of your opinions, since she doesn't actually want them. Being there to listen (are you familiar with active listening? If not, it would be good to become familiar, and to use it with her) is valuable for her in itself, even if you never offer opinions about anything.