an explanation for attending…which was because…

[llydonll]

Hey there, I am reading an English guidance book to write letters. Here are some sentences in the book.
"An explanation for not attending which was because my boss directed my team to work overtime to finish our project."
"As to the reason I am unable to visit you, which is because I promised my sister I will stay with her for Christmas holiday."

I think there are grammatical errors here in both sentences, namely "which" does not make sense. If I am wrong, what does "which" mean in each sentence? Thank you for your help.

 

[dojibear]

"An explanation for not attending which was because my boss directed my team to work overtime to finish our project."

This which refers to something in the previous sentence. So it is correct.
An explanation for not attending "that thing" was because my...

As to the reason I am unable to visit you, which is because I promised my sister I will stay with her for Christmas holiday.

This "which" refers to "the reason". But this sentence is incorrect. It would be correct if the first part changed:

Let me explain the reason I am unable to visit you, which is because I promised my sister I will stay with her for Christmas holiday.

Or it would be correct if "which" was changed to "it", also referring to "the reason":

As to the reason I am unable to visit you, it is because I promised my sister I will stay with her for Christmas holiday.

 

[llydonll]

This which refers to something in the previous sentence. So it is correct.
An explanation for not attending "that thing" was because my...


This "which" refers to "the reason". But this sentence is incorrect. It would be correct if the first part changed:



Or it would be correct if "which" was changed to "it", also referring to "the reason":

Thank you dojibear.