Congratulations on executing, launching and getting customers.
I expect you have a list of emails of previous signups(in a backup of your db), send out a personal email to each one thanking them for giving you guys a try, what did you like/not like about the app. What feature could we add or change that would get you to sign up again. Get some feedback and see if there is a killer feature you're missing that would gain/keep customers.
Try sending emails to pet sitters inviting them to join.
Is there a feature you could add that would benefit the pet owner, maybe a live chat, photo uploads, live updates on a status page that the pet sitter could add to and the owner could check periodically. Maybe drop brochures at vet offices and try to get pet owners to require their sitter uses your app.
You're probably charging too little . . . although Petsitters might have a lower comfort zone that most b2b apps but your sister was paying $99 so maybe $49 is a better price point.
I would second going with digital ocean. $5 to $10/mo plan would probably work easily for 8 total users.
Definitely think about it as a learning experience. Learn more about marketing, A/B testing, user engagement.
Track how often users are logging in, track users that visit the cancellation page and don't cancel (follow up with users who aren't engaged or thinking about canceling send them a personal email and offer a tutorial/on-boarding session so they are getting value out of your app.
Email each user that cancels and try to get some feedback on why they left, ways you can improve.
Listen to Startupsfortherestofus, lots of good info there.
Patio11 also has lots of great gems in his articles, HN posts and podcasts.
Good luck.