Don't change your plans. She was well aware that you had plans and you had advised her on Tuesday that you would be going out. It is up to her to locate a sitter if she wants to go out herself.
As for tomorrow, I would let her know that that your mother had invited all of you over on Sunday and if she wants to go, she is more than welcome. If she doesn't respond or says no, go and have a good time.
You can't rescue her, nor can you fix her every problem. She needs to learn that you will not be there for her every beck and call and that you are moving on w/your life. That means...facing the consequences of her own actions. Allow your w, to put her big girl panties on and grow up...do not rescue her as she needs to learn some necessary lessons that she missed out on as a teenager.
Have fun at the concert! Don't even thick twice about changing your plans.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
