Our bank holding company is in the process of acquiring another bank. Will we need to add those assets to ours to determine whether we have to move into the "Large" bank category, or is it only our bank's assets that count and not the entire holding company's?

Page 5 of the CRA Guide:

Who Must Report
All state member banks, state nonmember banks, national banks, and savings associations that are not small or special-purpose institutions are subject to the data collection and reporting requirements of the CRA.
For the purpose of collecting and reporting small business and smallfarm loan data, a small institution is a bank or thrift that, as of December 31 of either of the prior two calendar
years, had total assets of less than $250 million and was independent or an affiliate of a holding company
that, as of December 31 of either of the prior two calendar years, had total banking and thrift assets of less
than $1 billion. Institutions that are not small are considered large institutions.

It's only your bank's assets that matter. The holding company thing was taken out of the mix when the ISB concept was introduced. AR.

You would only combine the assets of the two banks (your and the one the holding company is acquiring) if the two banks were being merged/consolidated. As long as the holding company is keeping them as separate entities, you only have to worry about the assets of your bank.