Is the CPA right?

My mother is 78 years old. Last year, my brother and I decided to place her in a nursing home since we also have our own families. My mother has a co-op building. We have removed her name from the shares and lease of our co-op for estate reasons. Our CPA advised us that my mother should file a gift tax return since she gave her shares to me and my brother. So that we can determine the cost of the shares when the co-op was purchased. Is the CPA right? Should we follow his advice?