Not at all. Only when landing on an actual players main token, do you have to pay either the bank or the other player. Landing on any of their assistant disks does nothing. And of course, landing on their helper sends them to jail.
Yup, what Xhawk said... and keep in mind that you have to pay the merchant BEFORE doing the tile action, so you need to have enough money at the start of your turn to pay the penalty, or you cannot use the tile (you can just land there and take no action if you can't or don't want to pay). And if there are two or more merchants on a tile you want to go, you have to pay them all.
Also, the reward for sending a family member to jail is only collected after doing the action, during the encounters phase. You cannot use that extra money/card for the action on your current turn, and if you don't properly "enter" a tile (say, you don't want to drop an assistant, or don't pay the penalty to the merchant already there) then you cannot have encounters either.