Q. Did Esther have any brothers or sisters?
(You are asking why it matters) It matters because I found an interesting Gematria. In Ki Seitzei, where it says, Mishpat habechorah, equals exactly Esther or 661.

Why would that Gematria be the same for both phrases? Perhaps she was an only child.

A. After checking out the available sources in the Talmud, Midrash, Meforshim, etc., there seems to be no indication of Esther Hamalkah having or not any siblings. The Talmud (Megilah 13a) and Targum (2: 7) write that her father and mother perished immediately after she was conceived and was born correspondingly and did not have any more offspring.

It is important to mention that, even if she would have been the oldest daughter and had younger siblings, the Mishpat Habechorah or laws of the first born, would not apply to her.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a