Term for half sister. Protocols for writing

[rufus]

Hi,
Through research into my wife's parent's background we believe we've discovered a 'half-sister' in China that my wife never knew existed (my wife's parents and siblings are deceased and as far as she knew she had no other relatives).  Exciting, to be sure.  I have just managed to track down the person's address but I'm wondering if some advice could be offered. 
First, what term would be used when addressing her in a letter (she's older than my wife and is likely in her 80s)?  Would you address her as her name or would is there be a term for 'half-sister'?   Or conversely, what term would be used in referring to myself who will likely be writing the letter and my wife? 
Secondly, due to a number of circumstances (Exclusion Act, early death of half-sister's mother while husband was in North America, WW2, fear of being sent back to China due to 'paper' documents etc.) the existence of the the individual was never known to my wife. I worry that that the letter/introduction may be dredge up feelings of anger, resentment, etc. by the half-sister.    On the one hand it would be great to make the connection and to keep the memory of the ancestors alive but on the other hand some have suggested to just leave the past be.
If anyone else has gone through a similar experience any advice/suggestions would be appreciated.  Additionally, are there any protocols I should observe in the letter? 
Whether it's relevant to the discussion or not, I'm not sure, but at present I'm about 90% sure that the individual is my wife's half-sister.
Thanks.
Rufus

[lachinatown]

My two cents.

In Chinese, everybody calling everybody uncle, brother or sister. So use of these terms would be fine, like Uncle John, Brother Tom, Sister Mary. 姐姐 or 姐 (older sister), 妹妹 or 妹 (younger sister).

As far as connecting her, you have nothing to lose. If she rejects your attempt, then just forget about it. You have given it a try. If you don't, then you will never know. If UCLA Engineers didn't send that message to Stanford in 1969, would we have the internet today?

[rufus]

Good point!  Just concerned about dredging up potentially negative memories (on the other hand there could be some positive ones too!)
Thanks for the info on what terms are used.
Cheers
Rufus

[lachinatown]

For example: in your wedding picture (frontal one), it was addressed to 學洋弟 Xue Yang Di (the last character: younger brother). I would venture to guess that the recipient was not a real brother.

The character for the wife of mother's brother (妗).

[liuyao]

To offer some kind of encouragement, here's a story I just read today (no Chinese): www.dailymail.co.uk/news/article-3237565/How-discovered-guilty-secret-Titanic-violinist-grandfather-CHRISTOPHER-WARD-explains-bargained-uncovered-story-doomed-relative.html

[rufus]

Thank you lachinatown,
The translation helps a lot. The first two characters you refer to in the wedding picture (学泮) are the same as the name of my wife's father so I'm assuming that was whom the note was addressed, the other character (弟) for younger brother could possibly mean it was from my wife's mother's brother's family and refers to their child.  We also never knew about them and I've only recently uncovered their existence.
Thank you as well, liuyao.  It's an interesting article and certainly outlines the possibilities (however remote) that one can stumble upon.
I've now tracked down some possible addresses and have written both a potential 2nd cousin and possible half sister.  Now it's wait and see if they reply and what the outcome is.
Cheers

[lachinatown]

We agree with Woodson that the photo note was addressed to Mrs. Wong and her son.
Therefore if your wife's father is 学泮 Xue Pan, then Mrs. Wong was his mother. Would you know if this is true?

Don't get all wrap up with the use of 弟 (younger brother). In the United States, men are calling each others "bro". It is the same idea.
In other words, the writer of the note may or may not be a true brother.

[rufus]

lachinatown,
Thanks for the clarification about 'brother'  Too bad because it may have helped but more likely would have made everything more convoluted.
You stated,
"We agree with Woodson that the photo note was addressed to Mrs. Wong and her son.
Therefore if your wife's father is 学泮 Xue Pan, then Mrs. Wong was his mother. Would you know if this is true?"

My wife's mother (we've recently found out was a Wong; she came into the country under a different name).  As far as we know she did not have a son.    HOWEVER, her husband (my wife's father) did have a brother who died during WW 2.  His brother had a son.  He was referred to as a nephew, not a son in any correspondence that I saw.
Xue Pan, we believe, did have a daughter with his first wife but no son.
Do you see any clue in the information about who the older woman in the photograph is? 
Thanks
Cheers