Morally you are under no obligation, unless you have formerly promised your estranged child anything. As others have said, it is your money to do what you like with.
However, in your place I would take into consideration whether your estranged child has any contact with her sisters and brothers or not? If they have contact with her, then they may feel bad about her being cut out of your will. It might be best to discuss this issue with them, before changing your will.
My parents left everything to be divided equally between my sister and myself, to our great relief. Our maternal aunt, who was childless but close to my sister, left the bulk of her estate to my sister's children, although she had formerly told my sister that everything would go to her. She left me a legacy, but only keepsakes to her two other god-children, who my sister and I had known all their lives. We found the situation unfair to them and unkind, and as my sister was the only one of us all still living in rented accommodation, I personally felt she was being unfairly treated.
I think we have to face up to the fact that whatever we do, someone will be dissatisfied with our will! Explaining why you made the decision you finally come to, either in person to the children you do see, or in letters to them all individually could perhaps prevent some ill feeling.
I do hope you come to a decision you are comfortable with for that is really the most important point.