Boss, you are going to be a lawyer soon. Please learn to distinguish between offers and contracts. You did not sign anything yet. Nor have you accepted any signing bonus by way of consideration. You're free to do whatever you wish. Now if the law firm holds a grudge and doesn't let you in as a lateral entry later, then that's on them. Normally, they don't care much if you don't raise a hue and cry about it. Do remember to get a signed agreement from the second firm preferably though, before you inform the first one.

You need to send a mail to the RCC saying that u accept the offer and that makes it a valid and binding contract. The firm can sue you even though the chances are rare cuz it will cost significantly lesser to replace you than spending on litigation. Another problem is that word spreads very fast within the industry and you may just end up making a bad name for yourself at the very start of yo career.