Am I obligated to fill out the I-130 and why?

There never is a legal obligation to file a Form I-130 Petition for Relative Alien if the planned petitioner does not wish to do so. Note, however, that there is a very long backlog for visas in the Family-based Fourth Preference visa category - the category by which a U.S. citizen sponsors a sibling. While it may appear otherwise, that delay is likely to be 15-20 years or even longer, and the filing of the Form I-130 alone would not confer any right for a sister to travel to the U.S., to work in the U.S., etc. Instead, it is simply the first step in a process for attaining immigration benefits that cannot be completed until a visa becomes available in the applicable category. With this in mind, you might re-consider your reluctance to file a Form I-130. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.