We can see that what makes the sentence given above wrong is the addition of the word "is".
So, it should be: "I totally forgot that all of your cousins are coming, too."
What is sentence?
Sentence refers to a group of words which come together to make a complete thought. It usually have a subject and a predicate.
Thus, we can see that what made the sentence wrong is the introduction of the word "is".
Learn more about sentence on https://brainly.com/question/552895
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