How uncomfortable... Hint that you are for some reason not single, and see if her flirting continues. Some people actually don't realise they are doing it, or that it's such obvious flirting. I also concur with the suggestion you draw comparisons with your mother, just for clarification purposes. I can imagine if she continues the behaviour after you've made it abundantly clear it's inappropriate, there's a slight issue and I don't know how you could continue your friendship.