Ok, you need to take responsibility for dealing with this and stop pushing your husband to do what you don’t want to do yourself.
Whatever the rights and wrongs, it’s important to you that you spend time relaxing with your husband and children but it’s not as important to your husband.
It’s as wrong for you to insist he does what you want as it is for him to insist you do what he wants.
If he would prefer to be with you but can’t say no then that is his problem and only he can change that. You quietly pressuring him from the other side, while smiling sweetly and obeying his mother yourself, makes you as bad as her but in a different way.
What you can do in that case is be the role model you want for your children and demonstrate to him and them how to stand up to difficult people.
So, she’s going on holiday with you but staying in a separate location. Your husband is happy to meet her every day and you are not. You know her well enough to predict her actions. So, what are you, as a fully grown woman and mother, going to do to protect your family time?
You can be strong, say no and take the consequences.
You can arrange to meet once or twice and say no to everything else.
You can turn off alerts on your phone and only answer reasonable texts/phone calls.
You can just keep saying no to your husband and tell him to meet her himself if he’s determined to do so, but then you and your children still do the fun things you’d planned.
You know she isn’t going to frequent the places you will be so it’s only really phone contact you need to deal with.
There will be other options. So what are YOU going to do OP? How are you going to keep this holiday on plan for you and your children?