回复：请问一下：我想邀请我姐姐来美玩一两个星期，用什么理由比较

not very believable to VO if she says 玩一两个星期. Beware the possibilty of an annotation on the visa stamp page -- the applicant promised only to stay two weeks.
saying 玩一两个星期 will not increase the visa chance compared to stay for months, although if she has employment, she will also need to explain why she can be off for months.