The issue seems to be what Muslims believe about Jesus Christ. My question would be: "What does your sister-in-law not believe about Jesus that keeps her from becoming a Christian?
Scripture opens the door to the answer to this question through its teaching that God has a special love for children.
The only obedience that is necessary for salvation is yielding to the requirements of the gospel (John 3:16, Acts 16:30,31).
A few days ago a friend asked me to meet with her non-Christian family and share the gospel with them.
An orthodox understanding of the deity of Christ compels us to say he could not have failed in his mission. Indeed, if he could have failed, he is not all powerful and therefore he was not and is not God.