Why do Muslims

Where does the Quran confirm that the Holy Scriptures were changed?

We have the original Torah and the original Gospel.  Actually there are more inconsistencies in the Quran that can not be explained.  GOD for sure is not inconsistant nor incapable.

Your heart is closed to the fulfilled prophecies- that is the only true way to prove if a book is from GOD.  Otherwise all you have are the words of a man, no witnesses, although Paul did have witnesses.

You are correct in believeing that GOD does not have a wife, sister, brother.

Jesus as a manifestation of GOD does in fact have the One True United GOD, as His GOD.

Remember though that Jesus was 100% man and 100% GOD 

Where in the Quran does it say the Torah and Gospel were changed and altered?

minuteman- hypostatic union,  The divine nature of GOD attaches the human nature to Himself.  The divine nature of GOD wrapped in human flesh.

The human nature of Jesus did have a mother, and possibly a brother or 2.

minuteman - you really need to read the Gospels to understand.  Have you read the story about Jesus at the temple?

The Boy Jesus at the Temple

 41Every year his parents went to Jerusalem for the Feast of the Passover. 42When he was twelve years old, they went up to the Feast, according to the custom. 43After the Feast was over, while his parents were returning home, the boy Jesus stayed behind in Jerusalem, but they were unaware of it. 44Thinking he was in their company, they traveled on for a day. Then they began looking for him among their relatives and friends. 45When they did not find him, they went back to Jerusalem to look for him. 46After three days they found him in the temple courts, sitting among the teachers, listening to them and asking them questions. 47Everyone who heard him was amazed at his understanding and his answers. 48When his parents saw him, they were astonished. His mother said to him, "Son, why have you treated us like this? Your father and I have been anxiously searching for you."

 49"Why were you searching for me?" he asked. "Didn't you know I had to be in my Father's house?" 50But they did not understand what he was saying to them.

 51Then he went down to Nazareth with them and was obedient to them. But his mother treasured all these things in her heart. 52And Jesus grew in wisdom and stature, and in favor with God and men.

LOL!  your own Quran says He spoke from the cradle.