What if my daughter is lying......?

Be honest and upfront. As your family they should understand that you have to take your daughter seriously. All you can do is trust your daughter. My sister lied about my dad molesting her, a huge investigation took place and he was cleared. But years later when I really was molested by dad my mom chose not to believe me. I can not even begin to explain the amount of pain I felt when no one would help me. You have to take your daughters word for now, kids do lie but they also occasionally tell the truth. You could eventually forgive your daughter for lying a lot easier than she would forgive you for not protecting her.