I think it all depends on how much time you have to train her to go potty outside. If you work, and are gone most of the day, it's gonna be hard. If you are able to stay home and watch her, it shouldn't take too long. I have found that teaching male Yorkies to go outside is a lot easier than females. But then too, I have found that teaching more at once is easier for me. When my pups are old enough to go outside, they all go together. I have a fenced in yard, so they can't go anywhere. I do go out with them at first, but the more they go out, the easier it seems to get. I also don't use pee pads! All my Yorkies love to tear up paper. Whatt starts out as a nice little pee pad, almost instantly becomes little bits and pieces of paper ALL OVER THE HOUSE!!!! I just use an old towel. Get them used to going on that, then start getting it closer to the door. Altho sometimes I think they want to go out to do their business. I let them go out, they play and play, then rush back in and pee on the towel!! But, all you have to do is throw it in the wash and all's fine.
If I were you, I'd take all three of them. You never know what your cousins friend may do with the little girl if you don't. I live in CA or I'd offer to take her. I know a lot of people here that would jump at the chance.
Good Luck!
Suz