Personally I would just bury his ashes, then you can grieve properly. It does seem a bit weird, but don't let what your daughter-in-law did get to you. It is a shame about the children, not being allowed to know their grandparents. But you have to move on, If he was ill, maybe he had had enough but, it is just as likely it was an accident. Obviously if you really thought she had something to do with it, then you have to report your suspicians. Otherwise get on with your life as best you can.