I guess I'll be the OTHER vote. I love FB. I got rid of all the negative people. . . including my SIL. My FB newsfeed is happy, supportive, and kind. FB can be a great way to connect with friends and family, but, like anything else, it is what you make it. My FB newsfeed is drama free and inspirational. Like anything else, it is a tool. How you use it is up to you.
Now, regarding your situation. . . I'd unfriend yout SIL and keep your niece as a friend. Sounds like she will need your kindness and support that she isn't getting. You can be the difference for her and that is worth keeping FB for. Do not interact with your SIL no matter what. Once I stopped responding to my SIL, she started ignoring me, too. Ahhhhh. . . .peace!!!
Also, I keep the chat function off at all times. Yes, I agree that FB CAN be a source of drama and negative self talk. . . if you let it. Like I said, my FB is my happy place. You do have control of what you see and don't. There are some amazing recovery groups on FB, inspirational groups, and, whatever your interests are, many groups about that.
All that being said, your recovery comes first. So you may want to take a vacation from FB, then reestablish it as a healthy tool for you. It works GREAT for me.