Its my opinion that regardless of what we say or think on a regular day, we have no idea what we'd be capable of, or willing to do, in a crisis. You say that you don't know that you'd die for them. Of course, that's entirely reasonable, because today, your house is not on fire, your life is probably consumed with feeding and diapering your baby, trying to think of something creative for supper, and paying some bills. So don't label your willingness to protect anyone or anything in an emergency based on your adrenaline levels on a regular Wednesday.
And you are still post-partum. You're filled with emotions and hormones and love after giving birth to a child you never expected. Your natural mothering instincts are strong!
Lots of people declare, sometimes fiercely, that they love their foster, biological, adopted, or step-children the same. That's nice, and sometimes they do. But sometimes they don't. Often, they haven't been tested. Don't rely on what they are saying to judge by. That shouldn't make you guilty of anything. They're just more vocal, and have decided to make a statement. You may be more private, or not the type to make those public declarations.
Your statements, however, are loud and clear, regardless. Your actions speak volumes, and they're good. You call your boys "fantastic". You are kind and caring to them, and actively involved in their lives. That's what matters.
There should be no guilt, no "evil", no comparisons. Simply love who's in your life right now, and don't try to script some artificial statement about you love them all the same, while trying to hide your feelings. That will drive you crazy. Your boys are old enough to realize that a newborn needs more attention than they do, and that a mama snuggles her baby. At 7 and 11, they are probably typical boys, into sports or video games. Be truthful. Tell them how happy you are that they are in your family, if the subject comes up. They don't expect a declaration of whom you'd be willing to die for, and in which order. They simply hope to be treated kindly, loved with stability and security and consistency, and that is what they have.