Why did she choose 姐姐? Parents in China routinely introduce me to their children as 爷爷.  Okay.  Adults usually do not use family terms with me.  

There was one case that I wonder about.  In a fish restaurant where they had seen me before, I got my first squilla/虾蛄.  I was doing okay, but obviously not experienced with eating them.  The manager came over to my table and said 姐姐给你看  She did a very expert job, showing me at each step exactly how.  Of course I was pleased at her kindness.  

But I would like to understand better: She was probably 10 or 15 years younger than me.  Did she use 姐姐 because that is normal when we are somehow close to the same generation?  Or out of politeness so she would not be calling me old?  I have to say Chinese people (not only children) are often quite frank at calling me old.  Could she have called herself 妹妹, or would that be very unlikely between strangers?  Or would it be because she was taking the lead to teach me a skill?  Could she have called me 叔叔 or would that be wrong somehow?
She identify herself as 姐姐 to show she is “powerful” or “knowledgeable” in doing whatever you don’t know how to do. Sometimes you call a female 姐姐 for showing respect. Even for people younger than you, they may still identify themselves as 姐姐, because 妹妹 is too flirtatious. However, I don’t think it’s a good idea for you to call a female 姐姐 when you know she’s clearly younger than you (not in your situation, as she has called herself 姐姐). And, females don’t like to be called 阿姨 by strangers even though they’re much older than you, so 姐姐 is a safer bet. 

Another popular term to call young females (even though they might be younger than you) is 小姐姐, which has a lot meanings than it used to be. So the use of it depends on context, but generally I treat it as a nickname for young beautiful females. Pls be careful using it though. A lot of materials online you can search about.