I got to know that Korean grammar uses an extensive system of honorifics and those are used to differentiate between formal and informal speech.

So ways to formal speech is to honorific nouns and verbs.I was searching for these and found a wiki page which give some examples on how to use but I'm unable to find out :

• How to formally call 언니 (female's older sister) ? Can we use eonnim or (eonninim) ?
• Why the honorific for 오빠 (oppa) became 오라버니 (orabeoni) instead of oppani or oppanim ?

For the first one it has already ni suffix in eonni and that's why I guess there is no honorific, if this is the case how do call eonni informally or as a person who is on the same level/same social status.