Driving other's kids w/carseats

My daughter is 2 and is still rear facing. I have a niece that is 2.5 but smaller than my daughter. She's been forward facing since before she was 1 even though she didn't hit the 20 lbs mark until a bit after that.

I know that by the laws in Maine that my niece can sit forward facing, I would just prefer to have her rear facing if she's riding with us.

My niece has the same car seat as my daughter, so I know it's safe to have them rear facing to 50 lbs. Daughter is about 27 and niece is maybe 24 or 25.

Her parents just say do forward facing, it's fine, it's habit for her now. But my whole thing is that of course I don't plan on getting in an accident, but ya know they're called accidents for a reason haha

Should I have her forward facing with me because it's what she's used to or is it okay if I have her rear facing?