In the play 'An Inspector Calls", is Sheila the older sibling of Eric, are they twins, is she younger than him or is there no way for us to tell?

It was hard for me to tell, given that I haven't read it, however, according to what I'm reading, it looks like she's older than her brother, Eric.

http://www.gradesaver.com/an-inspector-calls/study-guide/character-list#sheila-birling