Sure thing

Oh right I can see why you feel the way you do then. Did you grow up with your sister or step sisters for very long or were you a bit distant?

I would definitely say its not down to you, from what I've gathered you're a very friendly and good hearted person so I can't imagine them having many negative thoughts about you. Perhaps though there's a negative connotation they have linked to you, perhaps your mother for example?

Do you have many close friends in addition to your ex-husband and dogs? To me family isn't necessarily about blood. Dogs are great additions to the family because of their empathy, I have two dogs and they comfort me a lot. I often consider my best friend to be like my brother, if you have people close to you, you could see them filling family positions too. For example a friend being like a sibling to you. If I were you that's what I would do, try to really solidify any existing relationships with friends or if you don't have many, try and make new ones and act as you would with your family, with them.