L.A.
I am so sorry to hear about this. I had a very good friend that was bipolar and it was very scary for everyone. She was either very happy or very angry. We as friends and neighbors mostly saw (90% of the time)the happy. But there came a time when my friend quit her medication and felt like she was doing fine. This is when it became scary. She needed major help and was not willing to accept it.
The thing to remember is that many people will not show signs of bipolar till their early or mid 20's. I am sure she did not intend to fool you in the beginning.
Please also know this is a disease. She has no control over any of this. It is not something she will ever "get over". She will need to be under a doctors care for the rest of her life. She should find someone she likes and trusts. She will need to take medications to help her system. There is no "cure". Your SIL should be in a managed program for this. There will be times when her medications will need to be adjusted, but she will fight it. It may also take a while to find the correct combination's, so she and her Doctor will need to decide if she should be under total medical care during those times, or if she can work on this from home.
This is part of that "for better or worse, sickness and in health". When you love someone it must be heartbreaking to see them so ill but just as if she had cancer, you will all need to be understanding and never let her stop taking care of herself. If you can assure her that you will always love her and be there for her, it may at least help her to not feel so devastated and shamed when she comes out of these episodes. Your husband and his young family should also go to counseling.
You may need to sit down with your brother and make plans for how he will handle these episodes and if any of you are willing, let him know how you can help with the care of their children or whatever may be needed.
I am sending you patience and strength.