Can that child come with my sister in the same file or not?

A U.S. citizen may apply for a brother or sister in the Family-based Fourth Preference visa category, but the backlog for available visas is so long that this almost never is a worthwhile immigration strategy (the backlog is likely to be around 20 years, even though the Visa Bulletin dates might appear to show a shorter time). It would be wise for your sister to consult with an immigration attorney who, after learning all of the relevant information about her, could advise about immigration eligibilities, options and strategies.