Good question, but I'd like to say that all depends on the person, family, the relationship between people.
Anyway, let's see the situations you gave:
- parents of your wife/girlfriend
It depends on the family. In my family, we use the formal way but not too formal, I mean, it's half formal half casual and it comes us naturally, little by little.
- uncles, aunts, any family member older (if age makes a difference?)
It totally depends on the family so you have to see which way people talk to but yes, the age would make differences. In my family, we use casual form but not to the family members of my husband.
- brothers and sisters of your wife/girlfriend (as old as me or younger)
It also depends on the person / family. In my family, the younger ones use a little bit, only a little bit formal but it seems that they use almost the same form.
- people on the street (younger, same age or older)
I use the casual form to the person who are exactly younger than me, like child, student...but who are same age, older, or simply to "adult people", we use the formal way.
- business relations on a private occasion (Karaoke anyone?)
Generally we use the formal one, however, it depends on the age and relationship between the people. You might be able to use the casual form to the ones younger than you on a private occasion but it depends on the relationship between you.
*You can say "please talk to me in casual speech" when you want them to talk with you in the casual form.
I hope this was helpful.