I'm sorry not to be very helpful here: I looked up Sette in my two very good
German namebooks - they don't list it. I googled it in
German, I tried
German wiki - nothing to work with there.
All I can do now is give you my opinion: Sette seems like a big stretch from
Roswitha to me BUT I live in a region where a
Matthias could go by Hirsl and everyone would know his real name just from the nickname (talk about a strech eh?).
So yes, there is a possiblity that Sette's full name was
Roswitha but as
Roswitha herself is so rare in Germany nowadays nobody would assume Sette to be a
Roswitha in her birth certificate ;)