Any parent will tell you that parenting is the hardest job in the world.
I'm here to tell you that the only job harder than that is parenting someone else's children.
Parenting, if done right, involves instilling young minds with values, morals, a code of behavior, a system of knowledge and beliefs. What happens if you marry someone who has children who arrive, baggage in hand, with someone else's values, morals, knowledge, and beliefs?
Don't think for an instant, now that you have revealed that your system differs from his system, that he is going to fall down in gratitude to you for showing him the error of his ways. The child will perceive you as an outsider, who's trying to come in and move him out of his comfort zone. How dare you?
Even if you're right and he (and the biological parent) are just wrong, wrong, wrong, don't expect the truth to be met graciously. Don't be shocked when they resist your ideas. Expect resistance. Mentally prepare for it. Truth is almost always met with resistance, initially. However, truth, like sunlight, never goes away. You can shut it up for a while but it always comes back.
Persistence is the key. Eventually wisdom will catch up with their emotions, and the children will discover which system works for them. You might even get a thank-you, some day.
Stick with it. Don't quit. If you're right, keep being right. The payoff will not come soon; it's far down the road.
But worth it.
In what truth will you persevere today?