Your husband is saying...nothing? It's his sister, he should have an opinion and you should be making this decision together, as a couple.
Speak to him again and come to a conclusion. Try to compromise a bit. Perhaps your children could miss whatever after school activities they have on that day, in order to complete homework before heading off to the wedding?
I understand that you have frustration carrying over from this woman's first wedding but please don't let that hamper your ability to take the high road and do the right thing. Do what you feel is right, setting a wonderful example for your children. Make every effort to make this ceremony and, if you have to leave the dinner early, so be it. At least you made the effort to be there. Another good example to set for your children; to make every effort to show care for your family members.
Let me know how it all works out. Top Wedding Questions Forum Moderator - "Write your sorrows in the sand, your blessings in stone".