[christine]

You still wouldn't say that your sister and me are going shopping.

i know, i know!! my sister and i are going shopping or my sister is going shopping with my husband and me. i am clear on that one.

the "younger than me" or "younger than i" is the only usage was not sure about. i understand that younger than i [am young] is understood. however, i thought perhaps that "younger than me" is also acceptable here. wish i had a dang grammar book!

[nopardons]

Okay, you want the " rules " ? Here it is, in English teacher language , thanks to " TEACHING ENGLISH GRAMMAR ". With the verb to be, the use of the nominative case is used, NOT the dative ( objective ) case ; in other words, it is correct to use " I " and incorrect to use " me " , in that sentence !

You sent me to the basement, in search of my old college grammar books . I haven't looked at them in many decades, but I DO know correct grammar usage. LOL In " ASPECTS OF AMERICAN ENGLISH " , the incorrect usage is shown by this example : He is stronger than me. This is the same sort of sentence ( replace " stronger " with " younger " , and there you have it ! ) I corrected. There are rules for all languages ; common, incorrect usage doesn't change the rules. : - )