What is the ruling on looking at pictures of women that appear while examining a video clip, for fear that there might be inappropriate scenes in it for my younger sister?

The default ruling is that it is not permissible to look at pictures of non-mahram women on electronic media without a valid reason. Therefore, one should refrain from following films that are highly likely to contain images of women, especially if they have a negative impact. You should also advise your sister and remind her of the impermissibility of films that include forbidden content.