Family visa, can I apply for a multiple entry one?

I am from the uk and have just married my Chinese girlfriend, it will take a little time to get my wife and step daughter a visa to be with me here in the uk so I would like to get a 6 or 12 month multi entry one so I can visit then whenever I want, I have 3 previous single entry ones in my passport, does anybody know if I can apply for the multi entry one or just keep applying for the single entry, thanks .