It is also hard for me to imagine a just turned one year old being able to reliease the chest clip on the MR, must have some good dexterity and strength!
Like Celtic said, ensure she has the harness properly tightened and the chest clip in the proper position.
Your sister can get some sticky velcro and stick a piece of the rought side on the clip. Apparently kids don't like the feel of the rough side, it hurst their little fingers so will hopefully deter her.
The purpose of the chest clip is to ensure the harness straps stay in position before a crash, so if the harness is otherwise properly situated and tightened, and she's not pulling her arms out of the harness or anything, then her little one is still safe in her seat. A chest clip being unbuckled is in no way the same as a harness being unbuckled!