My sister, being raised a Jehovah’s Witness, does not believe Jesus is God.  So she asked, “if Jesus is God, then who was Jesus praying to?”  Here is my reply:

You had asked in a text a few weeks ago “who was Jesus praying to?”  You thought the answer would be long and asked me to email the answer to you.  But realistically, the answer is quite short.
Jesus was praying to the Father.
Taking the entire Bible in context, Jehovah God is revealed to us in three distinct persons, the Father who calls us to Him, the Son (Jesus) who keeps us, and the Holy Spirit who sanctifies us (conforms us to the Son).
All the apostles recognize Jesus as God throughout the New Testament but none so plainly as Thomas when he sees Jesus’s and realizes it’s Him in John 20:28 “And Thomas answered and said to Him, “My Lord and my God!”
There are countless upon countless scriptures I can share with you revealing the Father as God, Jesus as God and the Holy Spirit as God.  We know there is only one true God (Yahweh).  We know the Father, Son, and Holy Spirit each have different roles yet they all have the same attribute as God.  Therefore, the scriptures reveal to us that there are three distinct persons within the one nature of God.
So who was Jesus praying to? 
God, the Father.