Hey little sister,
I’m not a plumber but I have some experience with my own plumbing. As to your question, whether you would have a sewage back up or not depends on top many variables like your elevation in regard to the sewage plant, sewage pumps in use or not etc. or
If you’re really concerned about the possibility, you can invest in compressible rubber plugs used to plug drains in areas that anticipate flooding. They do require the removal of toilet and sink drain fixtures to seal the drain. You might give a call to someone in the local water/ sewage department and see what they say.
hope that helps.