There's nothing wrong with that. But I just think when the twins were littler, younger than 7, MaryKate kind of had a little accent. She talked a little diffrent than Ashley. Nothing is wrong with that.
Like in the episode "Fuller House" where Jesse and Becky move to Becky's house, but then they decide to live with the Tanners in the attic.
Well, when Jesse was talking to Michelle (MaryKate), Michelle gives him her pink pig for a present. Jesse says "Are you sure, Michelle?". Then MIchelle says "Yes, I'm sure", she said it in an accent. She also used an accent in a few other younger episodes.

Do you understand? If you don't, forget it. It's not big deal.

I think you mean that Mary-Kate had trouble saying her "Rs". If you read my post entitled "Over Speaking" you would see what I am talking about that there as well.