Weird period time ??

So there's 3 girls in my house that get their period and thats how I track mine (before I got this but its a little late to see how different the start time is). It's usually my younger sister, my mom, and then me. But this month, it was my sister, then ME, then my mom a few days after I started (which also hasnt happened. We dont start when the other is still on). Im just confused and now I dont know when it's going to come. Is it normal to switch up like this? Could it be that since my sister is still only 13 and doesn't have a regular schedule its throwing me off mine? Or could it be something wrong with me?