Extension of Stay or Change of Status Nonimmigrants in the United States may apply for an extension of stay or a change of status on this application, except as noted in the Who May Not File Form I-539 section of these Instructions.
Multiple Applicants You may include your spouse and your unmarried children under 21 years of age as co-applicants in your application for the same extension or change of status, but only if you are all now in the same status or they are all in derivative status. You must complete Form I-539A for each person that you want to include on your application. If you need additional space, you may print additional copies of the Form I-539A to complete for additional co-applicants.
NOTE: Extensions granted to members of a family group must be for the same period of time. The shortest period granted to any member of the family shall be granted to all members of the family.
在我看来，I-539 filing instructions 的上下文里，”You may include your spouse and your unmarried children under 21 years of age” 的含义很清楚，就是「这两种人可以和你一起申请，其他人不行」。并不需要额外排除其他类型的关系，比如爷爷奶奶、八房远亲、同一个旅行团的张三、你在纽约街头偶遇的陌生人李四等等。要不然单独把这两种关系列出来说「这两种人可以」、其实任何人都可以，USCIS 是闲得蛋疼么？