I have two girls who are twins, and we had another child . When they were around 5-6 years old, we started talking to them about the new baby on the way. We explained that their brother would be a great buddy for school and fun times.
At first, they seemed excited, but also a bit worried. As it got closer to the birth date, we kept reassuring them that everyone was okay, and that the new baby wouldn't replace them, but rather bring more love and energy into our family.
After the birth of their brother, it took some time for them to adjust . They kept asking when I could "find" a nanny to take care of him so they could go back to just being with me. But as soon as we started including them in small tasks like feeding and rocking their little brother, they seemed to feel more connected.
Around age 7 or 8 was when they fully accepted their role as big sisters. They would help with bathing, diaper changes , and even got excited about being helpers during bedtime routines.
Now at ages 12 and 15, they have taken it a step further by helping with homework, reading stories in his bed before sleep, etc. There's been very little jealousy or resentment over the years. Our oldest actually just helped me schedule some activities for her siblings this Halloween and was excited about it.
In short, patience, open communication, and including them early on helped us create a positive experience for all kids involved.