That is a difficult situation you have & it will take time to heal from it. I have prayed for you & your family. But first let me give you a situation that Jesus had with his disciples when they asked Jesus a question.
John 9:1-3 As He passed by, He saw a man blind from birth.
2And His disciples asked Him, “Rabbi, who sinned, this man or his parents, that he was born blind?”
3Jesus answered, “It was not that this man sinned, or his parents, but that the works of God might be displayed in him."
This baby to be born has not sinned because of a rape occurring. Recognize that God ALLOWED this baby to be conceived, so that the works of God might be displayed in this baby & that you all learn to love this baby, given as a gift to you by God.
Please read Psalm 139 too. The baby is part of your wife, too. God allowed for the conception, so that God's works can be displayed in this baby & in your lives in recognizing this.
And what do you tell your parents? You speak the truth in love. And recognize God's hand in this. And remember another very important principle in Scripture.
Romans 8:26-28 Likewise the Spirit helps us in our weakness. For we do not know what to pray for as we ought, but the Spirit himself intercedes for us with groanings too deep for words.
27And He who searches hearts knows what is the mind of the Spirit, because the Spirit intercedes for the saints according to the will of God.
28And we know that for those who love God ALL THINGS WORK TOGETHER FOR GOOD, for those who are called according to His purpose.
I am THANKFUL that this baby will be born to two Christians--who will love this baby & share the gospel & by God's mercy this baby will be saved like both of you. May the love, peace & grace of our Lord be with you all.