A lot of readers of Big Girl, Little Girl/Be Careful, Onee-san! are assuming if the two MCs had a relationship the older one would go to prison. Without her co-worker, a huge liability, that's really unlikely. They could easily sneak around for 6-8 years, have the older girl adopt mai-chan, etc. etc. Japan doesn't scrutinize people for underage lesbian relations, you have to draw attention to yourselves. This one could even have had them carrying out sexual activity the whole time and still have worked out like it did. Hoshikawa Ginzu District 4 is actually pretty realistic.