▲你问的句子，是表示将来的“be+动词不定式”结构，不是“系表结构”。

关于这个问题，一般语法书都有论述，但大多不够详细。

▲在不同的语境中，“be+动词不定式”结构可以表示多种不同的语义。

一、表示“计划”或“安排”。

This line is to be opened to traffic next week. 该线路将在下周通车。

They are to meet at the gate of the school. 他们将在学校门口见面。

二、表示“命令”。一般译为“必须”或“不准”。

No one is to leave the room without permission.

未经允许谁也不准离开房间。

This naughty boy is to stay here until class is over.

这个俏皮的孩子必须待在这里，一直到下课为止。（老师惩罚他）

三、表示“可能性”。

Are you to pay the debt for your wife?

你能为妻子还债吗？

The lovely puppy is nowhere to be found.

这只可爱的小狗恐怕哪里也找不到了。

四、表示“意图”或“打算”。一般译为“想要”。

If you are to be there on time, you had better hurry.

你想要按时到达那儿，最好赶紧走。

If all of us are to remain friendly, we must be sincere with one another.

假若我们都想保持友好相处，我们必须以诚相待。

五、表示“应该”。

You have done so well. You are to be rewarded.

Such nasty questions are to be avoided.

六、表示“不可避免”或“注定发生”。例如：

Your plan is to be a failure.

The murderer was sentenced to life imprisonment and was never to see his family again.

She did not think too much of her unkind acts. However, they were to be harmful to herself, too.

Tom wasn't worried about his head injury, but it was to be troublesome for him later.

七、表示“期待”。

The actual use of the newly invented equipment is still to be determined.

Henry's appointment with the firm is yet to be confirmed.

八、表示与将来不符或相反的“假设”。

If it were to rain tomorrow, we would not go outing.

Even if you were to take a taxi now, you would not be able to arrive at your destination on time.

▲关于你提问的句子The musicians were to play jokes on each other as well as play music中，were to play jokes 应该理解为第六种用法，“不可避免做某事”。