Right now, everyone's emotions are pretty raw, tension levels are high and it is NOT a good time to be begging. But it is a time to be kind, especially if/when there is path to any communication.
And it is a time to develop a plan for your life. Get the DB books if you don't already have them. Find a solution based therapist. Take the Battery Intervention class. Take care of you. Do what you can, when you can, for your children, without a lot of fanfare, with all the sincerity that you have in your heart. Don't bash anybody- her sister, her parents, ANYBODY- they are doing what comes instinctively to them, which it to protect her from further harm.
You mention a brother. Are you close to him? He may be your only avenue for a while. But don't allow him to become your messenger. But let him know, in your general conversations, what you are doing with your life to turn yourself (not your situation) around. When he sees that you are for real, the message will get through, even in bits and pieces.
This is not going to go away quickly, so learn to be patient, even when it hurts and it's the last thing you want to do. It is hard lesson for all of us who are standing for our marriages to learn. But a necessary one, and one that will reap many benefits down the road. ( We live in such an instant gratification society, "let's fix things right now", but wounds of the heart take time. I hate to admit that, given my own sitch, but it's true.)
"Wait for the Lord; be strong and take heart and wait for the Lord." Psalms 27:14
Me: 53 H: 56 Married: 1998 S 25 (not at home) SS 25 (not at home) Sep 5/05- 8/05 Sep briefly 11/06 Sep 5/07 Served D papers 7/28/07
