Bottom line is its illegal for a 14 yo to possess cigarettes but not you - just like it is illegal for her to have alcohol but not you because you are a consenting adult. it is not a double standard it is a law. I can legally do these things because i'm an adult - bottom line case closed, my house my rules - children do not drink or smoke in my house.
I wouldnt buy it at all that they were someone else's. if she got caught holding drugs, stolen goods, anything illegal would it be a valid argument in a court that they were not her's. I would punish her in the exact same way as if you caught her with a cig in her mouth. If i could go back and do this all over again with my son when i caught him doing these "little things" i would have enforced consequences that would have meant something instead of believing all the bs that he told me and taking the blame because i have a beer now and then or i smoked pot when i was younger. none of this matters - what matters is your neice and my son are minors and they cannot do these things. when my son is 18 and has his own money he can buy his cigarettes and smoke all he wants - until that time its just no. dont let a child turn their consequences onto you.