I was（加个too) busy to _babysit_(babysit)my sister yesterday
题干有问题!如果不是too...to.结构就得说：
I was busy babysitting my sister yesterday
即：be busy doing sth
He was too busy to bother about such trifles.
他太忙,不能为这些小事烦心了.
I was too busy to spare time for a vacation.
我太忙抽不出休假的时间.