Why do some parents pronounce 妹妹 as měiméi instead of mèimèi or mèimei? I have heard some Taiwan (and maybe Mainland) parents call their daughters měiméi. I'm wondering what is the reason for that.
The pronunciation of 妹妹 (mèimei) could have been mixed up with that of 美眉 (měiméi) due to the popularity of Taiwan TV dramas where the Taiwanese cast tend to pronounce 妹妹 similar to that of 美眉, for example, in shows such as 《星星知我心》.

美眉 (or MM in short) is a term used to describe young and pretty women. This term is to 妹妹 what 葛格 (gége) is to 哥哥 (gēge). The 美眉 article on Baidu explains this:


  现在美眉作为常用语流行起来是因为八九十年代在大陆风靡一时的台湾电视剧（例如《星星知我心》）里面带台湾地区口音的国语“妹妹”的发音和“美眉”的读音十分相似，听起来柔婉可人，给听惯了标准普通话（在台湾地区又称“北京国语”）的大陆观众留下深刻印象，用美眉就能惟妙惟肖地反映出这个发音特点，当时70-80年代生人的年轻人不约而同地使用了这个词指代年轻美女，并借网络和新文学的力量使“美眉”的使用慢慢得到社会主流的认可。当然“美眉”也还同时表示原有的意思：“妹妹”，同理“葛格”就是“哥哥”。