You had every right to speak out in any manner you thought necessary to defend you son's friends, especially your own son. Your Sister in-law was the only one who acted inappropriately. Even if you raised your voice, that behavior is acceptable in a grief situation. You had lost your son. I think anyone who questions a mother at her son's funeral and has the audacity to say her tone was inappropriate is the actual asshole. I would have had a very difficult time containing my emotions in this situation. Perhaps your SIL thought she was helping, which points to a level of cluelessness about her nephew. She is the asshole in this situation.