Why use siblings on remplaced of sister? o when use sister o siblings? I am confused

Hi ! Because "siblings" means "hermanos" ; and "sister" means "hermana". For example:

1. My sister is the girl with red hair.
2. Let the siblings be united because that is the first law.

Note: it is more common to say "brother" than "sibling" to refer to two people born of the same mother.

• siblings es plural = hermanos (varones y/o mujeres)
• brother es hermano
• sister es hermana