Hard for me to tell my step-children that I love them

I am 31 yrs old and I am the stepmother to 3 children ages 15, 13 and 7 from my husband's previous marriage. Their father and I have been married for 3 years, together for 5.

The oldest lives with us, the younger two we have more than half the time. I love them and I do so much for them - attend ALL of their sporting events, school events, shop for them, run them around town, do school work with them, play with them, etc...so I feel like I SHOW them I love them, but I have the hardest time saying it. It makes me feel awful.

I notice it mostly when I am around my two nephews, whom I was there for their births and am very close to. I have no problems saying "I love you" to my nephews (or parents, or brother, or husband) - but I notice I never say "I love you" to friends or non relatives.

Why do I feel so uncomfortable saying this to my step kids when they are my family? Is this normal? It makes me worried that maybe I don't really love them deep down. I don't think I feel the same way about them that I do my nephews...in some respects I do - I do more for my stepkids, I'm around them more, and I'm just as protective of them as I am my nephews...but I just don't FEEL the same.

My step kids are very loving/touchy with me, which also makes me uncomfortable because I am not that touchy of a person...but they also don't tell me they love me very often...just every now and then - which doesn't bother me. I just feel like as the adult, it is my responsibility to tell them I love them and help make them feel comfortable enough to tell me if they want to. Any suggestions? Or any insight as to why this is so hard for me?