Princess Charlotte is jumping right into big sister mode. Ever since welcoming baby brother, Prince Louis, on April 23, the adorable 3-year-old "won't leave [his] side," Us Weekly reports. According to the outlet, "Charlotte is really enjoying having a younger brother to play with," and is "taking her big sister duties very seriously."

In fact, we got a glimpse of the siblings' bonding when the Duchess of Cambridge recently shared photos of the duo taken at Charlotte's surprise birthday party. In one of the heart-warming snaps, Charlotte plants a kiss on Louis's forehead while he's asleep, and yes, our hearts melted to mush.

Kate and Prince William threw their little girl a surprise birthday celebration at Kensington Palace after she was surprised with a cake to share with her classmates at Willcocks Nursery School. "They like to make a fuss on the kids' birthdays," the outlet reported. "There is always cake, her favourite dinner and a lot of presents."

According to the publication, Charlotte's "protective" personality is similar to her 4-year-old brother, Prince George. "Charlotte is just like him. They have a great bond, and a similar connection is growing between Charlotte and Louis." So sweet! We can't wait to see how their brother-sister bond evolves over the years. Especially since they're already wearing each other's clothes!