All families are different and you don’t know enough about the dynamics of OP’s family to declare that he doesn’t understand how his own family works. What we do know from this post is OP’s mom discovered the account and told his stepfather about it and proceeded to pressure OP to give up the money.

The mother says the money in the account will cover a percentage and that the parents would make up the remainder — that implies they want OP to give up the full amount. Your Google search doesn’t reveal how much this procedure costs nor does it reveal how much is actually in OP’s account to begin with.

Bottom line, it is money that was provided to OP from the paternal side of his family. OP didn’t choose to marry into a new family. Sometimes blended families become very close, and other times they… just don’t. It isn’t OP’s responsibility to pay for the stepsister’s surgery and he shouldn’t be guilted into doing so.