It depends on how she signed over the home.
If she removed herself from the deed, you and your siblings received a gift on that date. As a gift, your cost basis is her purchase price.
If she created a joint tenancy with right of survivorship. you have a 50% gift and 50% inheritance. You will average her purchase price with the fair market value of the home when she died.
I am sorry about the loss of your mother.
[edit 2/20/18 2:39 PST]
