Discovery may inquire into any relevant area of inquiry such as questions about conservatorship of the children, property, adultery, medical history, anything relevant to an issue which needs to be decided.
Discovery is governed by the Texas Rules of Civil Procedure. I suggest you hire an attorney to handle this for you. It's not really an area for do-it-yourself experimentation.
I am not intending this to be legal advice, because I don't know the particulars of your situation. Call me if you would like to discuss this or other isues.