Chester County Archives has records for 1714-1923. There are three sets of indexes. Which one you need depends on the type of file and the year the file was started. The year of the entry is the year that the first record was filed in the proceedings. This may be several years after the death of the decedent.
Siblings tend to be listed together.
The year of the entry is the year the guardian was first appointed. It is arranged by initials. Locate the section on the film for the first letter of the last name. Look at the chart across the top of the page for the first letter of the first name of the person you re looking for. The number underneath that letter is the first page of entries for people with that set of initials.