You would not have tolerated any of this from a friend, your mother is no different. If your mother cannot or will not behave decently then she should get to see none of you. Some parents really should not be allowed access to their grandchildren, they really should not.
No she is not lovely and your role here in this is one of scapegoat for her inherent ills. She has been conducting her own campaign of emotional abuse against you for many years.
It is totally unjust that you have been seen as the "difficult child" out of you and your brother, this has been entirely your mother's doing. You can break away from her control; this is really about power and control.
Your brother's role is likely one of "golden child" but that is a role too not without price. He is totally unaware of this price to be paid and his position though favoured is really a tenuous one too.
I would not let your dad off the hook either; he has played the role of bystander to perfection. He being weak too has acted out of self preservation and want of a quiet life. He has also failed utterly to protect you and your brother from her mad excesses of behaviour. He does not want to protect you from her mad excesses of behaviour either and would rather see you cop it than he.
Your mother has not been also above projecting her own insecurities onto you; it is really she that would start a fight in an empty room etc. You were not responsible, you were just a child. She failed you utterly as a parent and she is frankly a toxic grandparent as well to your children now.
She has trained you to be subservient, subsume your own needs and compliant to the point that you have no say when it comes to at all challenging her.
Where are your boundaries with regards to your mother; these seem to be far too low currently and she is taking full advantage of you basically not being able to stand up to her. You are deeply in fear, obligation and guilt with regards to your mother. Every cause for celebration you have had has been put down by her, she cannot bear to see you happy at all. Even buying something like a skirt (and your description of it is perfectly acceptable) is cause for her to put the boot into you. She cannot stand to see you happy and is wanting to keep you in the role she has cast you in. Your relationship with her is dysfunctional and she along with her enabler DH has been the cause of that dysfunction as well.
It is NOT your fault she is like this. You did not cause her to be that way and it is likely that her own childhood featured emotional abuse in it too. It is no excuse or justification for her actions now, I am just trying to further show you that you yourself have not caused her to act in the ways she does and has done.
Your DC may well "love" her but they are too young to realise just what is going on here. They are seeing you as their mother completely disrespected by her; they are really getting mixed messages here.
You are the parent and have the final say; they should not be going over there to sleepover; you have to protect them from malign influences. Do you trust her with your children, after all they are your most precious resource.
I would suggest you read and post on the "well we took you to Stately Homes" thread on these pages and read "Toxic Parents" written by Susan Forward. I would also consider seeing a counsellor to discuss this, it is important though that you find someone who has no bias about keeping families together despite the presence of mistreatment.