Bob with his sister often plays volleyball.这句话为什么用"plays"

因为 with his sister 可以放在句子的后面,而Bob又是第三人称单数,所以要用plays
（即：Bob often plays volleyball with his sister ）
若Bob with his sister often plays volleyball中的with换成了and,就应该用play了.因为Bob and his sister 是两个人,所以要用复数