How to delete a relationhip onInsertRow grid event. Need help

Could you please elaborate more on what you are trying to achieve ?

I need to apply some check before inserting row. If check fails,row needs to be deleted

If you want to check some property before inserting row, you can try using onBeforeNew event in Relationship Item type. 

Example:

In Part BOM item type I added below method in onBeforeNew client event. Similarly you can get the properties of relationship item and validate in onBeforeNew event.

var pState = aras.getItemProperty(parent.item,"state");
if(pState === "In Review")
{
alert("Part State is in In Review State. Cannot add new Relationship");
return false;
}
else
{
return true;
}

If you want to check some property before inserting row, you can try using onBeforeNew event in Relationship Item type. 

Example:

In Part BOM item type I added below method in onBeforeNew client event. Similarly you can get the properties of relationship item and validate in onBeforeNew event.

var pState = aras.getItemProperty(parent.item,"state");
if(pState === "In Review")
{
alert("Part State is in In Review State. Cannot add new Relationship");
return false;
}
else
{
return true;
}