This is an exercise from Longman Writing Series: 4. We have to correct this "stringy" sentence:
Last-born children, on the other hand, often have little responsibility, and they may be pampered as the “baby” of the family, but they are the smallest, and they have to get people to like them, so they often develop superior social skills.
How do I improve this? Here is my answer:
Last-born children, on the other hand, often have little responsibility. They may be pampered as the “baby” of the family, but they are the smallest, and they have to get people to like them, so they often develop superior social skills.
However, I don't think "BUT they are the smallest" works here.