I don't think the time is right to be responding to the SIL's text message. There will come a time when Ellie's message would be perfect...but right now, emotions are running high on all sides and it will open up the battle of the texts. When dealing w/emotional inlaws that have only heard one side, it is best to let things settle down before even attempting to defend yourself or try to explain things.
WR, you know your inlaws better than we do and you need to trust your gut on this one. If you opt to send a response, please be prepared for the likelihood of more nasty grams.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.