Pray this prayer sincerely. It's called my loved ones Needs Salvation
Father, my heart burdened for ### because he/she has not accepted Jesus as his/her savior. He/she is trying to live on this earth without a relationship with You. I can't stand the thought of him/her spending eternity in hell. I know that You have been long suffering toward him/her because You are not willing that any should perish, but all should come to repentance (2 Peter 3:9). Notwithstanding, You said that even though all souls belong to You, the souls that practices sin shall die (Ezekiel 18:20) and be eternally separated from You. O lord, please send someone across ###'s (in your case your in-laws) path to minister salvation to him/her. You said that no man can come to Jesus unless You draw him (John 6:44) Prepare his/her heart now; do not let him/her resist the courting spirit by Your spirit. I take authority over the stronghold of sin captivity ###'s (in-laws) mind. Let him/her find no comfort, joy, or fulfillment in his/her life of rebellion. Help me to let my light shine before him/her so he/she will see the joy, peace, and other benefits of salvation. I pray You will fill ### with the knowledge of Your will through all spiritual wisdom and understanding, and he/she may live a life worthy of You and may please You in every way—bearing fruit in every good work, growing in the knowledge of You. (Colossians 1:9-10). In the name of Jesus I pray amen.