If you're the type of person who loves Disney (like us Dis'ers) then it is worth it.
But if you're going with the thought that this is your
first and last trip, you've already closed your mind to your having fun - In other words, it won't be worth it.
If you're not a Disney lover, then it will never be worth it. There will always be something wrong with something; too expensive: I could've bought that at the cheap souvenier store on International Drive for more than half the price; the hamburgers cost more here than a whole meal at McDonalds; The candy was cheap; the characters weren't out when I wanted them out; I had to wait on a line for five minutes but the Dis'ers said there would be no line;
We've heard it all here on the Dis. There is nothing a Dis'er can say to convince a non-Dis'er if something is worth it.
I can tell you that me and my family love
MNSSHP. It is definitely worth it to us. I'd rather not waste my money on a hundred pairs of shoes and a new outfit every week. I'd like to spend it on going back to Disney.
When it comes to dealing with your SIL, don't try to convince her until you are blue in the face - it won't be worth it. You'll be frustrated and she'll get tired of hearing you.
Since you have the tickets, go and have the best time that you can. Tell your First And Last Time SIL to keep an open mind. Disney is for everyone, and MNSSHP is Disney Magic at its Finest!!!