Not only my father but also all of us _____ looking forward to meeting my uncle.

就近原则
1.由连词either...or,neither....nor,not only....but(also),not just...but,not...but 等联结的并列主语,谓语动词要和靠近它的主语一致,如:
Not only you but also Mary is going camping tomorrow.
Neither you nor I am for his suggestion.
Whether you or I am chosen is uncertain.
2. 在there be 句型中,谓语动词常和最靠近它的主语一致
There is a pen and some books in his schoolbag.
There are some books and a pen in his schoolbag.
3. 楼主仔细看我给出的例句1，当第二个主语是单数时就可以用is了

就近原则

就近原则。从us，
同理，如果改为一般疑问句就要用is了

楼上是对的``就近`

are,就近原则