I'm so sorry this has happened.
I've a little understanding of what you are going through, as a close family member was involved with growing marijuana (But not my child) and suffered the consequences.
I can clearly relate to the fear, anger and devastation you feel.
I do think you need to talk all this through with a counsellor.
For your sake and your sil and gs.
If as CafeAuLait suggests your DIL is feeling overwhelmed (and who knows, embarrassed?)
She cant cope with your feelings as well.
Yes she and your son should have your perspective at heart but they feel they have bigger fish to fry.
When you feel you have created a calm space in your mind ( with the help of counselling) you can offer your services and she may ne able to invite you back into her world.
Dont blame your self. You are going through a kind of bereavement. You have to face the loss of who you thought your son and dil were, and face up to who they actually are.
Clearly they have chosen different lives to the one you and your partner have.
Dont forget you have a choice to make, if they do get back in touch. Can you live with who they really are, or is it a life you would rather leave behind?
Youve been forced into this situation, so at the moment you are in shock.
Be kind to yourself and try not to think in terms of blame. But if you do, let those feelings come and go.
The most important thing for now, is your health.
Get any support you can. Look for groups on line, you maybe able to vent your feelings to someone who understands exactly what you are going through.
Good luck.