I honestly don't know. Well, let me qualify that. I think you can, but maybe not everyone does. Do I? I can honestly say I don't know, and I only have a DSD and no bio kids. Ok, correction, I have another SD and a SS (13 and 17) who live with their mom, SO's ex, and I know I don't feel the same about them as I do about younger SD because she is mine full-time where I only see them now and then. I am her parent, but not their's. I think it totally depends on the relationship and your involvement with the child. I love SO's other kids, but no, I don't love them like I love their sister. I've been in her life since she was about 1 1/2 and living with her and SO for nearly 4 years. She's almost 7. Her mom died when SD was 13 months old so I am it. The only mom she's really known or will remember (even though she will be sad, even cry, and say she misses her mommy, but that's another post and more questions about how much she should have been told and at what age and how many photos and mementos should have been given to her and at what age...) and aside from all the "firsts" and milestones of the first year, I've been there for and been involved with all the major developmental milestones of her life. And I want to be there, and expect to be there, for the ones to come. Menstruation, first kiss, prom, graduation, college, marriage if she chooses, and kids if she has them. I hope to be there for her for all of that. And I hope that she and I have a good relationship and she always wants me there and thinks of me as her mom (she does tell friends I'm her step-mom or her mom) and though she'll be sad her birth mom isn't there, I hope she feels the joy of having me there. I never wanted children of my own, and being a parent, especially a step-parent who also has to live in the shadow of a much beloved and missed birth mother, has at times been unbelievably difficult for me. I didn't have any experience and, as much as I hate it, a troubling lack of patience for a lot of it. However, I love her very, very much, and I WOULD DIE FOR HER in an instant. I would protect her with my life if needed and she is a priority in my life. Though I have often found myself in tears or angry enough to punch things and silently wishing I'd never met her father because I can't stand parenting, I know deep down that I love being her mom. Being that SO and I are not married, I am not sure what legal rights I would have to see her, but I also think, or hope, that having been in her life for so long and in the capacity I have been, that if he didn't want me to see her he'd have a hard time upholding it in court. SO and I have had our troubles, and I almost left him a couple of times. Would I have wanted to see SD if we had broken up? Yes. Are there moments when I think no? Yes, but I know those are moments of stress and anger or sadness when I am not enjoying parenthood. I also know that she would want to see me and would be devistated if I were not in her life, and that alone would be enough reason for me to do my best to continue the relationship. But as for your original question, I don't have any bio kids to base this on, but I honestly do think that if I did there would be a connection, a bond and honest emotion that is not there with SD. It's actually one of the reasons I don't want a child of my own with SO. I sometimes think I wouldn't mind having a baby of my own, but there are many reasons NOT to and one of them is I would be afraid I would, without consciously meaning to, treat that child differently and SD would feel it.
I'm sorry for your situation. It sucks and it's not fair to the kids.