I know for fact that I have several other step-sisters and probably a few step-brothers via my biological father, so that is a very valid point about including at least a letter. I don't believe that the DAR accepts DNA testing as of yet, if ever, but I would be willing to have it done for the sake of a more complete application for someone else down the line.
Paternity was never an issue and he has always acknowledged me as his daughter. I would never be willing to have him placed as the father on my birth certificate, just because it is my step-father who raised me as his own and was there for me, and it is him that I call my dad, so it is a matter of respect for him that I would never have that done.
However, personal feelings don't count for DAR lineage, and it is only the biological ancestry that counts.
I have at least three Patriot Ancestors, Thomas Ansley, Peter Rape II and Samuel Carrol Handley Sr. that I am aware of currently, and I likely have a few more via my mother's side of the tree. While that wouldn't prove direct lineage for my step-siblings, since they descend through my biological father's side of the tree, my main interest is to submit the most complete paperwork possible and as such I think a letter would be a good idea.