Who are Horus and Isis?
They're two of the most prominent deities from ancient Egypt. Horus was the god of many things especially related to the king and the sun (but he wasn't the only god related to the sun). Isis was Horus's mother. She represented maternal protection and the power of royal women.
These two figurines of Isis would have originally included small figures of Horus as a child who Isis would have been nursing. The headdress you see on the left is in the shape of a throne making a connection between Isis and Horus's right to rule.
Why does she have a throne on her head?
The shape of the throne was used as the hieroglyph to write Isis's name. It was one of her symbols and a way to easily visually identify any image of a woman as the goddess Isis. The throne is a symbol of royal power, think about how the king sits on a throne. A king's mother's royal identity can be a big factor in his right to rule. Isis, as a legendary queen and mother to a legendary king was considered the origin of the right to rule.
What was the purpose of these sculptures? Were they important people?
These figures all had some kind of religious significance. Isis, represented in at least two of them, was one of the most important goddesses in the ancient Egyptian pantheon.
The larger limestone figure is also identified as a goddess, but without an inscription, headdress, or other defining attributes, so we can't be sure which one.
The headless figure is also difficult to securely identify, but the scepter she holds suggests that she represents either a queen or a goddess.
Why do they hold their chest?
The headless figure is simply holding her scepter close to her body, a useful feature in sculpture to make sure parts don't break off.
The two busts of Isis would have actually included a representation of her son, the god Horus, as an infant nursing. They are holding their babies to their chests.
I am looking at the two busts of Isis. On the green colored sculpture, why is there a rectangle shape on her headdress?
It is in the shape of an ancient Egyptian throne (if you look at it from the side, you can see that it is stepped), and represents how important Isis is to the king in terms of ancient Egyptian mythology.
It is also an easy way for scholars to identify a sculpture as depicting Isis.
Do all Isis sculptures usually have the throne on their head?
Not all, but many! In the myth, Isis was the wife of the legendary first king of Egypt, Osiris. When Osiris was murdered, he became the king of the afterlife and his son with Isis, Horus, became king of earth. Osiris had already died with Horus was born so Isis raised her son alone.