How bad was the sprain? ^^;
Let's put it this way.. assume this happened during the school year. Kumoricon is a little under 2 weeks away. Would your mom expect you to miss over 2 weeks of school because of a sprained ankle? You might need to limit your walking, but I really imagine your sprain will be doing a lot better after just a week. ^^;
This is just my opinion. Don't take it to mean "OMG I SHOULD GO!! FORGET WHAT MY MOM SAYS!!" Just something for you (and maybe her) to consider.
Ultimately though, your safety is #1. If you can't do it, you can't do it.