totally agree with you aromulus
bit of advice, make sure your stepdaughter is in the uk and allowed to stay before she is 18, once shes 18 its near impossilbe to get her a visa to stay Permanently.
and getting a visa for a step child can be a long process, we were luckly that my wife still was in contact with my stepsons bio dad, and he gave us a letter explaining he had not supported or seen him much, and wanted his son to come and live in the uk with his mom and sister, so getting a visa for my stepson was not a problem, but for some people it will not be that easy.
if i remember right, if your wife is on a 2yr spouse visa, your stepdaughter doesnt have have to be in the uk 2yrs, unlike your wife before she can apply for ILR.
i did the same with my stepson, he was in his final year at school, and thought it be better if i got his aunt to live with him while he finished school and went to college, but becuase she had family problems she returned home, and by then he was 17, i got him a visa and he is here in the uk, but hes now turend 18, and iam applying for ILR for my wife, daughter and him, but its going to be really difficult to get it for him... if i had known this i would have brought him with us 2 years ago,but then it wouldnt make much difference, as he would still have been 18, before my wife could apply for ILR.
i'll be phoning a couple of law centres near me asking advice, before i make a ILR app for him.