I used to think a little like you did when I was young. Second weddings should be small affairs, maybe a court wedding and backyard reception and no big white dress or lots of attendants. You got your wedding already, so you do not deserve to have another big wedding as a second one!
That was until I got divorced. HA! Now I had to eat my words!
I made some mistakes. My current husband, who had been married before, had made mistakes too. Both of us had nice first weddings. But now we REALLY met the person of our dreams and we wanted to celebrate this new beginning with the wedding we always wanted. Both of us threw out the pictures from our first weddings. We cherish the pictures from our wedding. It's what we show our children and will one day show our grandchildren. We look back on the day as the celebration of our union and I'm SO GLAD that I didn't have my younger self telling me that I couldn't have the wedding I wanted! White dress? Did it! Big reception? Did it! Attendants? Did it! Registered? Did it! I do wish with all my heart that I could have had bridal showers because I missed hanging out with my girlfriends and all the stuff we had been using was from our first weddings and it would have been nice to pick out shower gifts for OUR wedding. But we did not have showers, and that's okay.
Yes your stepsister might be going overboard in your opinion. Maybe she's just excited about finally doing it the way she wanted with the man of her dreams. I think every woman has gone a little bridezilla about her big day at one point! The wonderful thing about her wedding day is you probably won't hang out with her at all! She'll be too busy!
I'd go and hang out with your family. Dance with your husband to the free music! Enjoy the open bar! Eat free cake! And buy her a nice gift that fits your budget. You'll be SO GLAD you went! And I'll bet she'll be happy to know you cared enough to celebrate with her :)