Hi,
Seems like she's pulling the strings in "her" family, and they don't realize this. She's being controlling and manipulative of the members of the family, and you are one that she knows won't put up with all that, so she makes you to be the bad guy. She has probably always been this way and no one has confronted her and made her accountable for her actions. They basically ignore it and sweep it under the rug like it never happened.
She had already created the chaos that she wanted, thus it was time to "forget the whole thing". That leaves you feeling unsatisfied and confused and resentful, which is where she wants you. She knew that it upset you because you made an honest and sincere attempt to find out what was wrong.
You might need to start using some non-defensive sentences to her attacks. For instance, when she's blaming you for not bringing the kids to see her, reply with "I'm sorry you feel that way". That puts the ball in her court. As long as you continue to defend yourself to her, she will use what you say against you, and it makes her even feel more in control (even when she's not).
You need to understand that this is the way she is (your husband told you this also) and that she's been doing it a long time, and that she knows her children and other family members are not going to say anything to her. She does not care. On the other hand, you seem like a very caring person, and I know that you haven't done anything to deserve this. It is not you, but her. Once you understand this, you can move to changing how you relate to her. Please don't drive yourself crazy trying to figure out the "why" of it all. There is no rational why.
In an ideal situation, your husband should go straight to his mother and have a talk with her about making these kind of negative comments about you to other members of the family. He should talk to her about her disrespect towards you. If she doesn't comply, then he should let her know he will not attend family functions ( or whatever ) until she can act like a decent human being. He chose you and she should respect that, and he should demand that she, at least, respect that. She doesn't have to like you, but she should respect you. Showing disrespect for you is disrespecting your husband, her son. What does that tell you about her? She will only care about herself.
Read Susan Forward's book, "Toxic In-Laws". It has very good ways to deal with these kind of MILs. I didn't realize there were so many mothers out there who still want to control and be overly involved in their adult children's everyday lives.
Please take care of yourself and read that book. Hope this helps some.