Although this may feel a bit awkward, it’s not insurmountable. To me, it shows that this man raised a good son. You and your daughter both see something in this family that is appealing.
I’m impressed that you and your friend have waited to make sure you are serious before involving the kids. This is wise, regardless of your current situation. Now that you want to move it to the next level, by all means be straight with the kids. You kept it from them for good reason and you can tell them so. Having decided to continue, you certainly don’t want to be role-modeling that sneaky behavior is okay.
One caution: Be sure to caution the kids that because you and your boyfriend are moving forward, it doesn’t mean that they are locked into their relationship with each other. They are very young. Chances are they are going to change as they grow. It’s highly unlikely that they will end up together. Indeed, I hope they will go through a number of relationships before they settle on their forever person. One thing to talk about is how they can enjoy each other’s company now without creating a situation that would become very uncomfortable for them if they were to eventually become stepsiblings.
Conversations about the advisability of waiting for sex are definitely in order. Forbidding it would probably go in one ear and out the other. But, please, 14 and 15 is really too young to get sexually involved. I suggest each parent have a frank conversation with your own child to talk clearly about the implications of being sexual at this point in their relationship. Remind them that the rules are different for people who are in their 40s. Urge them to wait at least a couple of years, both to give themselves time to mature a bit more and to see how the current situation sorts itself out.
I wish you well.