This is a very tragic situation to be going through and my heart goes out to you, your husband and your extended/blended family.
As difficult as the situation is for you, you realize that your stepsons mother and siblings aren't really in a place to consider your feelings as they're probably not 100% sure what to do with their own. Grief and sadness is a terrible monster. At this point, you should just try to be there for your husband and your daughter-in-law as it does not seem that she's requested that you go away.
With your husband, it's different because you're with him and you can be extremely supportive without outside influences. With your daughter in law, when you have a chance and you know she's not with anyone else, simply tell her that you're there for her in anyway she needs.
With regard to the funeral, you're right, unless you're asked don't offer any suggestions or volunteer to assist with the planning. You can share any thoughts that you may have with your husband and if he decides to share them then that would be okay.
There is never really a right thing to say during this time, offer your condolences. I wouldn't say that you don't know what they're feeling because as you said, you do but that's not really what they want to hear right now. You have nothing to apologize for and your support is paramount. They may not appreciate it now or ever but I'm sure your daughter-in-law and husband will.