I'm thinking of fostering a child against my daughter's and ex wife's wishes
Why would you want to foster a child against you daughter and ex-wife's wishes?
To show them that I can be a good foster parent.
Fostering a child is a good thing to do, but it is a large responsibility and something that should only be done after a lot of careful consideration. You can't bring a foster child into your family when the rest of your family members do not approve of the decision. You should only go forward with fostering when you have come to an agreement with the rest of your family.
I don't need their approval for any of my decisions, I can do what I want.
This sounds like a difficult decision that you should talk over with your family and really give it more thought. The foster child could be negatively affected by this and it's not fair for them. Examine why you have this drive to foster a child and if it's more about helping the child or satisfying your needs.