First, I think it sounds like SIL is competing with your daughter. Trying to insert herself with her as someone older and wiser, trying to put her in her place in her opinion. Its clearly immature, childish, and pathetic, as you have mentioned how immature they are. However, you need to rein them in. Dont take the bait to get into name calling. Be a grown up. Protect your daughter from attacks.

Second, speak to your daughter. Its time she realizes that Facebook is public, that the things you say can follow you forever online. Not to be baited into arguments, to put all of her emotions, opinions on there and expect people to be decent and honest in return. She needs to know that there are many many people out there that will see what she cares about, her feelings, opinions and emotions and use them to get what they want. Some people are just trolls, some people are manipulative and will use her in worse ways. She needs to know that people that are bad dont all have cloven feet and tails. She needs to back away from facebook being the platform for sharing her inner most thoughts like its a diary. People will use her. DONT tell her to let it go because they are family. You dont want her to think that just because they are blood related that verbal abuse and attacks are acceptable and that you have no power over that because you are related.

Lastly, I would make it clear to SIL and Bro, that if they attack your daughter publicly again you will shred their ass like hash brown casserole for every one to see, cut them off, and hold them accountable for the fake "religious" leaning while hurting a child. Extra points if you say it calmly, quietly, and a big smile on your face. Make it clear that SIL behavior reflects on your BRO, and he will be held accountable as well, for hurting his own niece. They may be childish, immature, and petty, but those characteristics of people you would want in your life if they werent family.