One of the chief benefits of the Gospel is that God the Father adopts us as His children through Christ, His co-eternal and co-essential Son. Since Muslims do not believe in the Son of God, they neither know God as their Father nor do they worship the Father. Muslims will sometimes deny this and pretend that they believe in and worship the God and Father of our Lord Jesus Christ, but those who know better say otherwise.
Since Muslims believe they are brothers and sisters but deny that God is their Father, the million dollar question is: "If all Muslims are brothers and sisters, then who is their common father?
Either Muslims must reckon themselves to be cosmic orphans, something Jesus said was not true of His followers (John 14:18), or they must own up to the fact that they are sons and daughters of someone else. But who could that be? As the old saying goes, this means they are stuck between the Devil and the deep blue sea.