You are faced with quite a dilemma. If you find out she is not your biological daughter, you will no doubt feel a variety of emotions that could complicate your relationships with family members. If you do not find out who the real father is, then your relationship with her may become contaminated with doubt. This could easily be acted out in negative ways towards her, her mother, and even the other children.
We would recommend that you decide whether or not you wish to find out whether or not you are her biological father, which the court in most cases, can direct via genetic testing, if either party refuses to participate on their own.
Needless to say there are legal issues of importance here, mainly waiver and estoppel. These issues must be discussed in detail with the attorney representing you.
Regardless of the paternity results, you would do well to see a counselor or therapist to process the feelings you and your wife have about this issue.