The Tiger

You should realise the difference  between how someone is addressed and what you call him. For example you might speak of someone as “Mr Brown” but address him as “Sir”. Which brings us to the very sensitive issue of how to adress your mother-in-law.

“Old Girl” is not such a good idea under any circumstances.

Usually when it is concievable and well possible you might address her as “Mother” and talk of “My mother-in-law”, but it is really not practical when her mother is not that much older than you that is when you sit with the dilemma. The fact is in principle you could even be her age or younger. No logical reason why not? Such a woman would feel terribly insulted with “Mother” or “Aunt” scary forbidden.

When she is one who looks after herself and appearance and youth is very important what now? Or would address her as “Ma'm, Mrs Etc' or Miss”, being a man getting on best not it would be fatal obviously just first names and “you” but that would sound disrespectful in some circles if you go strictly according to tradition. Such customs are there for a reason.

Her father might not like you calling him by his first name, so things now can get a bit confusing. You might be uncomfortable with that yourself. Most men are relaxed more as far as about it even “oom”  as “uncle” could possibly work under circumstances, say if her mother had re-married but “dads” would mostly be acceptable. 

Anyway chances are he would be a few years older than you. You could always use “sir” but “hey dude” is a bit too casual. Don't forget there will problems from the bride's side as well, and if there are grandparents I think things can get really complicated.

The only solution is simply that everybody irrespective just use first names and “you”, but most people would not accept such a thing out of respect and tradition. In practice it would take years even decades at best for things to change.

As a rule it is a good idea to be in her good books or if she can tolerate you at all, especially when things go very wrong. When you decide to marry a girl the prospective mother-in-law should certainly be taken into consideration after all fact is you are marrying her as well. 

This all is only theory I don't talk of personal experience, but it is a problem that I have seen, and do anticipate.