Your Cost Basis for the stock would be her Cost Basis on the Date that your mother added you to her account. If there were three of you on the account at that time, then only two of you when your mother passed away, this gets complicated.
You would need to calculate the Cost Basis for the number of shares you had when your mother added you to her account, and another Cost Basis for the additional number of shares you received when she died (which would have a Cost Basis as of DOD).
If the Broker issuing you the 1099-B can't help you with this, you could enter the 1099-B as you received it, especially if the time period between her adding you to her account and her date of death are not too far apart.
If they are, you may need professional help to calculate this.
Click this link for an example using a home of Calculating Variable Cost Basis.
[Edited 3/10/2020 | 4:51 PM]
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