Ah well, Ezio (the MC) is just laid-back as a character and he considered Meredith to be that annoying bigger sis, so he just really let it slide (until he couldn't anymore a few times - Remi's talk was one of them). The revelation would come soon and that might surprise a few folks, including you too.
Trust me - if you can bear with her during her worst state, you would get to enjoy her at the cutest state known to mankind! :D