How should I handle this situation with my nieces?

My sister just gave birth to a baby girl yesterday. She already has a 4-year-old daughter. The 4-year-old was the first born, so she received a lot of attention, and was a bit spoiled. Now with the new baby, the 4-year-old daughter is having meltdowns and feels like she isn't loved. When I go to meet my new baby niece, how should I handle the situation so my 4-year-old niece doesn't feel ignored or replaced?