Your mother deeding the house to your sister would generally be a gift from your mother to your sister that would require you mother to file a gift-tax return, Form 709. Similarly, if your sister now adds your name to the deed, that would be a gift of 50% of the house from your sister to you that would require your sister to file a gift-tax return. In either case it's unlikely that any taxes would be owed, but filing Form 709 would be necessary to show that.
With regard to selling the house, your mother's cost basis in the house transferred to your sister with the gifting of the house to your sister and if your sister gifts 50% to you then each of you would have 50% of your mothers basis in your respective 50% shares of the house. Either of you to getting an exclusion for any of the taxable gain upon the sale of the house would require meeting the eligibility requirements: https://www.irs.gov/pub/irs-pdf/p523.pdf
