Help i have a weird relationship with my stepdaughter and stepson?

Well i am 31 and i married my husband who was 34 at the time. I was 26. But from a previous relationship he had two kids one is a daughter who is know 18 and a son who is 14. but i am 31 and there step mom. and to begin with that my husband had his first kid at 21 he is know 39. And it a weird relationship because i they were my real kids i would of had them at 13 and 17. so how can i just don't feel weird or out of place around them.