There's a St
Sylvester who died on 31 December, and in German-speaking countries they call the 31st Silvesterabend (=
Silvester's
Eve, just as
Christmas Eve is the day before Xmas) so it's a bit tenuous unless you speak
German.
Out of your name generator list, I'd prefer
Nicholas (but not
Nicola) or
Friedrich or
Karsten to either
Leopold or
Amadeus, especially as the only nn I like is
Leo and it's getting pretty popular as a name on its own.
Wulf seems a bit fluffy, but maybe
Wolfgang might work, as a link to the Mozart family.
I think if you used
Silvester as a fn and your sister used it as a mn and didn't call her child by it, there wouldn't be a problem and the cousins would have a nice link.
Actually, I've just noticed that if you spell
Leon backwards you get
Noel! A possibility?