I agree -- it's time to move out. The very generous arrangement with your sister was for college, which is now over. You should now tackle your own life on your own & let your sister's family lead theirs. It's not about your convenience or finances. Don't assume they're fine with you staying because they haven't said anything. Even if they beg you to stay, which I very much doubt, it's time to be responsible for your own life.
We allowed our nephew to live in our vacation home so he could concentrate on his studies at a local community college for a year or two to transition to a 4 year university. Two months later, he quit his part time job. He went on academic probation twice, making the university path unlikely, so he wisely switched to a 1 year certificate program for technicians in tv, film, & radio. Three summers after he moved in, still no job or certificate, sleeping until noon, & playing video games to all hours. Meanwhile, our son, 2 years younger, was working full time during summers & attending full time university the other 9 months. Realizing that we were enabling our nephew's behavior, we told him it was time to leave. We were surprised at the relief we felt -- we'd been unconsciously tiptoeing around him & getting angry. A year later, he's 25 & living with his parents, still with no job & without contributing -- why not, it's easy. It's for sure not what an adult does. We really feel sad & concerned for him.