OP, while I do not agree w/your stance on this issue and think you are being a little too
judgmental regarding your friend, it is
your prerogative in life who you do and do not want to socialize with and if something this person does makes you feel uncomfortable, then you have every right to not spend any more time in her company.
That being said, if you already have committed to this trip beforehand, I suggest you go ahead and go. I understand on your end it won't be too pleasant being around this woman who probably makes you
extremely uncomfortable now, but since the rest of your family is already looking forward to this trip and you have already agreed to go, I would just go. Try to have as little contact w/your friend as possible and maybe she will get the hint.
After the trip, then you can cut off any friendship ties you have w/her.
People will most likely want to know why, if you can...You can confide in your husband because he probably will be confused why suddenly you do not want to spend any time w/this woman.
It will be tough since your daughters are such good friends however. You may have to just "suck it up" and agree to be at least civil for the girl's sakes.
I am guessing your friend confided in you about her fling because she had a lot of guilt inside and needed to tell someone.