Wolf:
It seems more complicated than that.
Firstly, devarim she’b’leiv einum devarim, so if you manifest kiddushin it is irrelevant what you actually thought.
Secondly, if the couple was “assukin b’oso inyan” (of marriage), the gemara says we assume the money was given for kiddushin and accepted for kiddushin.
So, you were presumably not talking about marrying your niece. If you were, it would present a similar difficulty.
I think the main question here probably is whether we can say it was not kiddushin when everyone knows it is not kiddushin despite that the classic manifestation is present.