I get where your coming from Sarah, I don't think it would matter what sex the doll is it may be good for Zander to associate it with his new sister so he can role play alongside you but in saying that he will probably do that with a doll of either sex, have you tried taking him to the shops and seeing which one he leans towards or maybe if you attend a playgroup or find a friend who has some dolls you can try before you buy and see which one Zander prefers,

goodluck and I hope to hear what you decide.

We have a friend who has definate ideas about gender and was really horrified when I put one of Paige's dancing skirts on his 1 and a half year old boy (a nice hot pink and sparkly number) he totally loved it and danced around watching the sparkles, dad declined the offer of a loan...can't imagine why????
Spent the rest of the night talking to him about the importance of letting children role play and experiment with toys and allowing them to engage in open ended play with materials(the child carer in me I guess)