Answer:
The best way to combine these sentences is as follows: Sandy skied into a pine tree and broke her leg.
Explanation:
As the two independent clauses that make up this compound sentence share the shame subject (the NP "Sandy"), it is possible to ommit the explicit subject of the second clause. That is, to write "and broke her leg" instead of expressing "and she broke her leg" overtly. This is possible when you have a compound sentence with a coordinator such as the one above (and), the absence of said coordinator would make the sentence ungrammatical as the second clause would lack the subject (see 3 & 4). You can only ommit the subject shared by both of the clauses when they have been joined by a coordinator. What is more, in this case, you need a coordinator or special punctuation, otherwise you create a run-on sentence (5).
3. *Sandy skied into a pine; broke her leg.
4. Sandy skied into a pine tree, broke her leg.
5. *Sandy skied into a pine tree she broke her leg