I know your sister was sincere, and I respect her sorrow and her desire to remember and honor her son–but no, we don't become angels when we die and go to heaven.
The truth is, when we die and go to heaven we become even greater than the angels! The angels are spiritual beings who were created by God to be His servants, and God has given them great authority and power to do His will. And at the present time the angels are greater than we are, because God made us "a little lower than the heavenly beings" (Psalm 8:5).
But the Bible also says that in heaven we will be higher than the angels–and the reason is because we will be like Christ. The Bible says, "Do you not know that we will judge angels?" (1 Corinthians 6:3). While this verse may be referring to the rebellious angels who chose to follow Satan, it still implies that our status after death will be above that of the angels.
All of this, I know, may seem like theological hairsplitting to you. But don't lose sight of the central truth: When we know Christ, we know that this life is not all, but ahead of us is the joy of heaven. May your sister take comfort in this truth, and in the fact that her little son now is beyond all the pain and suffering of this world. Is this your hope as well? It can be, as you give your life to Christ and trust Him as your Savior.