How do we look inside the cell?
Techniques to look at whole cells are: Light microscopy (cells, large organelles), electron microscopy (detailed analysis of subcellular structures and even proteins) and confocal fluorescence miscroscopy (look at particular cellular planes, reconstruct 3D images).

And of course you can analyze the insides by Biochemistry by breaking the cell membranes and look at individual proteins and DNA/RNA by extraction and electrophoresis followed by Western / Northern / Southern blotting, or isolate organelles by centrifugation.

It is a broad question, and I just gave the most obvious examples of techniques (not anywhere near exhaustive). By googling the bold-out terms you may provide some details to your sis.