Here's why:
* "Between" is used for two or more items. It suggests a separation or distinction.
* "Yourself" is a reflexive pronoun. It refers back to the subject of the sentence.
In this case, you are referring to two people: your sister and yourself. You would need to use the reciprocal pronoun "each other" to indicate the relationship between them.
Correct usage:
* Between your sister and *each other*
* Between your sister and *you*
Example:
"There was a lot of tension *between your sister and you*."