Hi & welcome to rootschat
Do you have the years of birth & forenames of your brothers or was your father not in touch with either of them or their parents?
I would imagine the woman who was unmarried would have registered the birth in her own maiden name & likewise for the married woman, the birth registered in her marital surname.
Did your father tell you what districts the boys were born & were they both in the same district?
I have no idea how the law works in Chicago but in Scotland we can visit the General Register Office to look up births (at a cost).
Is there somewhere you can do those look-ups or get someone to do them for you if you have enough info?
I will just remind you, no living people's details can be given on forum but after 3 posts you can co-operate with others via Personal Message.
Annie