Should I go to the father's day breakfast in my sister's first grade class?

I've been her guardian since our parents died four years ago. At the beginning of June, her class is planning to invite dads for doughnuts and coffee in honor of father's day. They did the same thing on Mother's day, and even though mothers were not required to go, I still heard that she was very sad when she had nobody there to celebrate with. I don't want her to feel the same way for father's day. I'm kind of a father figure since I'm her guardian, but I'm still her brother. Would it be appropriate for me to accompany her on the father's day breakfast? And I've tried asking her, but she won't give me a straight answer, so I don't know how she feels, and I doubt I ever will.