I think it mostly depends on 2 things. Whether they will agree to it, and whether that is the extent of the activities, or whether you wish to take it further.
Money is a great motivator, and if you want either of these women to pose naked for you, money is probably the best way for you to get your wish. Asking them to commit sexual acts with each other is taking things to a whole different level. You might argue that a kiss is just a kiss, but when you are talking about 2 naked women kissing each other, there can be no doubt that there is a sexual component to this, and a mother and daughter are very likely to refuse point-blank, unless they are exhibitionists, into lesbian incest. A pretty unlikely certain circumstances, I would imagine. Again, offered enough money, most people will do just about anything. It depends on whether you have a big enough bank balance.
The unasked question, which is really what we would call 'the elephant in the room,' is where do you expect this whole situation to go in the end? One thing that is clear, the young girl likes you, and is already on intimate terms with you. In fact, she couldn't get much more intimate without physical contact, and at 19 years of age, she is at her sexual peak of interest.If you are an unscrupulous type of person, there is little doubt that you could get your own way with her, but these things all come with consequences.
So, is photography all you want? And if it is, why does it have to be in the nude? Are you taking these pictures for art, or so that you can look at them while masturbating? Personally, I think you need to get out more, rather than weaving intricate fantasies about your maid and her mother.