Do you know granmaagain, I think grannyactivist has truly hit the nail on the head.
Your D's responses make perfect sense and are not about a deficiency in your mothering skills - be proud that she dealt with the situation on her own to start with (she would have been wrong to broadcast something that would seriously disturb family relationships if she was hoping to get it sorted and get back on track) and be aware that the upbringing she received gave her the strength to do that, and the strength now to feel her own sense of worth despite her sadness and the rejection she has suffered.
Pat yourself on the back and continue being the excellent loving mother that you have always been. This is what she needs. Be sad, by all means, but do not turn inwards and agonise about your mothering, rather ackowledge that very understandable sadness, and gird up your loins and be strong for her.
Treat your SIL with courtesy - he is a father in this situation and that role is still important - and try and think how you would have treated a son of your own who did such a thing. You would not stop loving him, but you would be extremely disappointed in him. People don't always live up to expectations and he has failed everyone, but he has not suddenly become worthless - he has made a huge mistake, but he is still worthy of your care and concern - you did not care for him wrongly - he simply failed and made a huge mistake through human weakness, which we all share.
I really do think that things will shake down into a new pattern of life, and that your role as a strong rock to lean on when needed, and to stand back when not, will be crucial. Take up that challenge and move on.