Will I have to pay his child support once we are married? Can they take my wages?

What about when we buy a home---presumably it would be in both of our names once we are married....could the ex-girlfriend use that as an example of increased ets on our part and thus go back to court to increase the monthly support payments? We have two children together as well...my family insists that that this is one of the reasons that we should stay unmarried---SO that I won't end up having to contribute for children that are not my own out of my own wages. I am working hard in school to increase the quality of life for my children. She only has a highschool diploma, is on welfare, and recently popped out another kid with another man. It is not my business the support determination that my fiance has BUT it is my business when we start talking about me putting my hard work into paying for her to get her nails done. Will our income be looked at jointly once we are married? Will our tax returns be seized for any arrears(arrears are easy to have when one parent is on welfare)?