Honesty is always best for me... and usually the hardest.
You could write back, and thank him for the card. It takes a lot to reach out... sometimes more than we realize.
I would probably advise you tell your sister you love her... just because. I lost a brother that I wish I had told that to.
You might tell them how you want a relationship with them to look... go slow? By letter only at first? Whatever makes you feel good. They may never 'get' what they did... and you have to decide if you can live with that.
You sound like a strong, caring woman. I hope this works out for you.