My husband's extended family is very angry that I wont attend a family wedding by myself?

Well, the bride seemed happy enough if she sent you back a note and something for your son. That's all that really matters anyways, tis is her wedding, not her aunts wedding. I see no reason you need to attend. The reasons you should share with your family off your list are you have midterms, your son has therapy, and finances won't allow. Do not share any other reasons, as they could be seen as offensive. But those 3 reasons should be good enough.