Your attempt is close, but it doesn't fit the pattern expected from you, I believe.
I will either attend the party, or if I can't my sister will.
When you put "either" after "will", you essentially specify that there are two (mutually exclusive) actions that you will take. The description of the first follows the "either", and according to the use of "either-or", the description of the other ought to follow the "or". For instance,
I will either run the 10K or swim the relay (but not both).
He will either reach the top or die trying.
In other words, the exclusive portions each follows the "either" and the "or" in the sentence. If, as you describe, you might attend (one portion), and in case you're unable, your sister will, then you need to wrap those into parts to follow the "key" words:
Either I will attend the party, or, if I am unable, my sister will.