To be honest, I really can't think of a visa that she can qualify for if she isn't planning on working or going to school for 18 months in the US. Also, this is probably not an issue for British nationals, but I have been told that it is sometimes difficult for citizens of certain countries to obtain a non-immigrant visa to visit the US (especially for a long time) if the applicant has a sibling who is a US resident/citizen.
Hopefully, someone more knowlegable would be able to answer your question.