Since your lg is a PhD student, I guess that he has scholarship/fellowship/stipend/TA/GA. Check international office if the money is more than to support both of you. If so, you don't need provide any extra money. If asked, tell VO 陪读. For 在职身份签探亲/graduation, you should appy B1/B2 (you can change to F2 once you were in US).