I am researching this question today, and found your post. I believe you get a step-up basis, but I cannot tell you what percentage of the account receives step-up basis. As of this writing, probably either your mother's 50% of the account gets a step-up, or the whole account may get a step-up.
You should consider having a tax professional review your particular situation. If you have filed the return with the incorrect basis, then you want to look at filing an amended return before the deadline to receive any additional refund you may be due because of the step-up basis.
I found this at the California Estate and Elder Law, LLP site:
"If the joint owner who died paid for the entire property, the full value of the property is included in the deceased owner’s estate. The property receives a 100 percent step-up in basis. For example, if Mrs. A owned stock, put it in joint tenancy with her daughter, and subsequently died, the full value of the stock would be included in Mrs. A’s estate. Her daughter would then inherit the property with a 100 percent step-up in basis.
If both joint owners contributed to the value of the asset, the value of the deceased joint owner’s share is included in his or her estate. That portion of the property receives a step-up in basis. If Mrs. A and her daughter bought stock for which Mrs. A paid 60 percent and her daughter paid 40 percent, then 60 percent of the value of the stock would be included in Mrs. A’s estate and would receive a step-up in basis."
