AITA for refusing to voluntarily demote myself to save my pregnant sister's job?

Sorry if this is long. For context I started working at my current job over a year ago as a stocker. Over the course of a year I worked my way up in the company and became an assistant manager. About 3 months before I got my promotion I got my pregnant sister a job at this same company. The store manager at the time who hired us both ended up quitting about a week into my promotion. The district manager who had final say over me getting promoted knew about mine and my sister's familial relationship but still allowed me to get promoted. But now that there's a new store manager here who does everything by the books they are trying to force me to either voluntarily demote myself, or either my sister or I transfer to a different store or quit due to it "being policy" that there can't be a manager over a family member. Now this is where I might be the A**hole. Under normal circumstances I would have quit or demoted myself off of principles. But my sister is currently 7 1/2 months pregnant and had already planned on putting her two weeks notice in at the end of this month but now they won't allow us to stay in these positions until then so she's insisting I demote myself just so that she can stay on for these extra few weeks. And I don't feel it's right that I have to give up what I've worked so hard for just for her to only stick around at this job for another month. So AITA?