Forming sentences with resultative and directional complements When studying Chinese, I've often found that there are different ways to say the same thing, for example using a verb with a directional complement rather than a verb with resultative complement or a simple sentence structure. 

I was told that the choice (prefering a directional complement over a base sentence structure) is due to a change in focus, thus with a directional complement the main event within the sentence is a movement taking place.

So, if I have these sentences:

(说话人在上海)
1. 我妹妹到上海来了. [using the directional complement]
2. 我妹妹来上海了. [using the simple sentence structure]
3. 我妹妹来到上海. [using the resultative complement]


Are there any differences between these three sentences?

Also, I was told that the third one is wrong, but why exactly is it? Couldn't it be interpreted as a resultative complement where 来 is the main verb and 到 indicates the result of arriving at a place?

Thank you.
Yes, there are indeed differences. I will mostly give a discussion of in the example you gave since I'm really not competent enough to talk about the complements.


In (3), we have 来到, which means that 我妹妹 must have arrived (or is arriving). (Notice that the lack of a 了 means that without more context, I cannot infer whether or not this is in the past tense.) In any case, there's a slight difference between this and "my sister came to Shanghai": we can still mean "arrived" by "come", but the connotations should be noted.
In (2), we have 来 ... 了, which merely indicates to me that 我妹妹 must have come to Shanghai. There could also be connotations of arrival, again dependent on context. There could also be connotations, because of the 了, that 我妹妹 has now left 上海. (As an aside: I would group 了 with the verb, instead of separating them.)
Like (2), (1) feels ambiguous. I could read 到上海来了 as "has come to Shanghai", and like in (2) it could be that the sister has just come to Shanghai (so has arrived), or merely "she has come to Shanghai" in a more general sense. However, unlike the other two, (1) gives me the sense that Shanghai isn't just some intermediary stop for 我妹妹 because of the placement of 到. The other two can be slightly ambiguous in this respect.