Implementing marginfigure

Implementing marginfigure

sorry for the absolutely beginner question here.
I've been looking at the tufte-latex class, specifically the sample book (found here). It makes use of marginfigures, which I believe are provided by the sidenotes package.
As you can see in the book, there is a marginfigure implemented with the code:
\begin{marginfigure}
  \includegraphics{file}
  \caption{caption}
\end{marginfigure}
However, in my custom class file I use the sidenotes package using
\usepackage{sidenotes}
It should be noted that this works fine, I can use normal sidenotes without any problems.
However, when I go to put in a marginfigure using the code above I get the following error:
Environment marginfigure undefined.
\begin{document} ended by \end{marginfigure}
How can this be fixed so that my marginfigure implements properly. Sorry for the basic question, but I hadn't found any other fixes.