Is the sentence "Mother made sandwiches for her and me." ungrammatical?

No, it's not ungrammatical

Explanation:

The sentence "Mother made sandwiches for her and me" is correct and follows the rules of grammar. The personal pronouns are used as subject (I, you, he, she, it, we, they) and/or object (me, you, him, her, it, us, them). In this sentence, the use of the pronouns "her" and "me" is correct because they are objects of the preposition "for". " ... for her and me (us) ".