I think you're right, 2 people cancelled and now she has tix for you.
Tell your husband to deal with this. It's his sister.
Your niece is 18. Call her and let her know that. Tell her that you and your husband would like to do something special for her just the 3 of you sometime before the graduation party.
I think your niece not only will understand but needs to see the big picture. She's old enough to handle it.