Both images were snapped by the duchess at Kensington Palace.
"This image was taken on 2nd May, on Princess Charlotte's third Birthday," one picture is captioned.
It shows Charlotte holding her sleeping newborn brother and tenderly kissing him on the head.
Another photo, taken on April 26, shows Louis alone and with his eyes open. He was born April 23.
"The Duke and Duchess would like to thank members of the public for their kind messages following the birth of Prince Louis, and for Princess Charlotte's third birthday," reads a message accompanying the portrait.
William and Kate, who have been married since 2011, also have a 5-year-old son, Prince George. William is the son of Prince Charles and the late Princess Diana, and the grandson of Queen Elizabeth II.
The Duke and Duchess of Cambridge are very pleased to share two photographs of Princess Charlotte and Prince Louis, taken by The Duchess at Kensington Palace.— Kensington Palace (@KensingtonRoyal) May 5, 2018
This image was taken on 2nd May, on Princess Charlotte’s third Birthday. pic.twitter.com/H5VVgIwRGp