My daughter does this too...she's 5 and her brother is 3. There are times when she is totally fine and makes good choices, etc. and there are other times that she acts like a maniac and does "3 y/o" things and yes, it also drives me bonkers....
But I don't think it's attention issue, I don' think she feels left out or anything that complicated. I truly think it's all about having a sibling and seeing what they do and wanting to do it too, no matter the age. It's almost as if they realize, wow, I didn't even know that was an option...I wanna try that!
I would say if she's doing something that is inappropriate that you are also trying to correct the 2 y/o for, then yes absolutely discuss it with her. Or if she's talking in a baby voice or crying to get something she wants, as her 2 y/o sibling might, then yes, again, correct her...but everything else, eh...I'd let it go.