What's in a name?

I've been asked to call my host father samjan meaning "uncle." This is different from opa (older brother) that I was calling him. Turns out samjan feels opa is meant for someone younger than him...oh, and apparently, some girls call their boyfriends opa. Needless to say, I'm more than happy calling him samjan. However, I'm still addressing my host mother as oni meaning "older sister."