Even if you place your sister and family in a private tree, searchable or unsearchable, once you connect any Ancestry records to her or her family, those records WILL be offered to anyone and everyone researching the same people via the Ancestry hint system.
The following will help:
1) Use first name initials only, and leave last names out (you know who they are, anyway)
2) Do not use their exact date of birth, use only year (exact dates of birth can easily pull public record hints for others to follow)
3) Do not connect any Ancestry records to them, period