Just be true to who you are. You can send cards or do whatever you want to do. You can't control their reactions and what they choose to do is no reflection on who you are.

I may have missed something, but it's possible that your sister in law was trying to respect you. She might have thought that you didn't want to hear from her. If there's a relationship you want to maintain, you could try sending a letter saying that you just can't live with your husband, but you aren't divorcing the family. You hope they know you love them and for the children's sake want to stay connected.

I'm so sorry for your pain. I know how hard it is.