I am not an English teacher but I believe that “US” relates to “WE” and the correct wording for that pronoun is “WE ARE”. On the other hand “ONE OF US” relates to “I” and “YOU” but the correct wording for those pronouns would be “I AM” or “YOU ARE” … There absolutely is no place in English where we correctly use “I IS”, “YOU IS”, or “WE IS” so why would “US IS” be correct?
Your sister appears to be correct. ‘Neither of us’ is understood as ‘neither one of us’ and is treated as singular. From the Commnet "Guide to Grammar and Writing":
The pronouns neither and either are singular and require singular verbs even though they seem to be referring, in a sense, to two things.
Neither of the two traffic lights is working.
Which shirt do you want for Christmas?
Either is fine with me.
According to Ngram, it is also the most conventional usage.