You asked about the use of "relating" and "related" in the following sentences. Please note may comments in parentheses:
1.The report is related to the accident...(Correct)
2.The report is relating to the accident...(Not Correct)
3.I will prepare A report related to the accident.(Correct)
4.I will prepare A report relating to the accident.(Possible , but the -ing form is normally used for something that is/was happening at a particular time or that is true all the time,neither of which is the case here)