Firstly, I’m so sorry for your loss. ?
I think that your SD is probably feeling a whole range of emotions right now, including guilt and possibly regret and has given vent in part in that letter to you.
At the moment you are trying to cope with the death of your beloved husband, so perhaps now is not the time to respond, if at all, (and my own instinct would be not to respond at all, as SD made clear her feelings previously by breaking contact with your DH and if you have any, her step siblings).
You might feel, as time passes, that you would like to respond, but think very carefully if it would be the wise thing to do.
The cynical side of me wonders, like Calendargirl if she thinks she might be entitled to some inheritance and is making contact, albeit angrily, in the hope she might have been left something?
Tread carefully would be my advice, and follow your gut instinct. As you say, it could possibly open a can of worms - would you be able to cope with that?