Legal Question in Family Law in New York

No, you would not be entitled to share in your fiance's separate property (property owned before the marriage) unless you have contributed to the increase in the value of the property. Unless he transfers property into your name, you will likely not be entitled to 1/2 of his accumulation.

I would supplement the previous answer only slightly:

If you are divorced you are entitled to an equitable share, presumably half, the value of any bank accounts that are in joint names, regardless of the source of those funds. You are entitled to a share in the increase in value of property he currently owns ONLY if you contribute to that increase.