Yes you can also talk about ex in-laws here

I am sorry you are feeling frustrated, hopefully getting your story out here will help
Recently she really triggered me. She contacted me to tell me that her son had seen my daughter posting about drug use on social media and wanted to know what was going on with my daughter.
When was the last time you had contact with her? Does she contact you regularly?
When I asked my ex sister in law if I could have her address so I could send her a postcard she ignored my request and I haven't heard from her since.
Are you trying to have some sort of at least cordial relationship with here so they can be a part of your daughter's life? Was that the reason you wanted to send her a postcard?
I can't tell you how angry it makes me feel that my ex sister in law thinks she can just text me anytime she wants but has no obligation to respond to me.
Has it always been this way that she doesn't respond to you or are you just talking about this one occasion?
My ex sister in law as well as my ex mother in law have been mean from me to the start however 20 years ago I did not know that kind of behavior was a red flag so I didn't stay away. Now I know better.
Being treated meanly isn't pleasant at all so I understand why this would bother you. In what ways have they been mean to you? Could you give some examples?
Once we know better, we can also do better. How they behave is up to them, but how you let them treat you is up to you. Also for dealing with hostile ex in-laws, boundaries are essential.
Furthermore my daughter called her cousin a snitch and that really broke my heart as previously my daughter used to adore that cousin. He was the closest in age to her and as my daughter is an only child he was like a brother to her.
Does your daughter still have contact with her cousin or had their relationship already become strained before this incident?
Take care