Sorry this will be long but I'm very passionate about this and I know you don't know my backstory.
Coming from someone who techinically only has half-siblings (I don't call them that.) My oldest sister was raised by her BM and was parental alienated against my dad. In this case, the other kid was my dad's so my parents had pictures of her hanging up all over the house that relatives would send him. Her BM ran off to Mexico with her for several years but this was in the 70s and not much could be done in terms of enforcement of visitation back then. Anyway, she was never hidden from me, but we were hidden from her. BM had illegally enrolled her in school when they moved back to the states under her SF's last name, told her we never wanted anything to do with her, ect. I finally met her when I was 10 and she was 20. She found out and located us and had to go back and have all her school and driver's license records changed to her REAL last name. That said, her BM raised her and my sister harbored a lot of resentment towards my father because she loved her mom. She stuck around for a few years and then decided that she didn't feel like my dad called her enough (he doesn't call anyone) and cut us off. When I turned 20 and had my DD, I contacted her again once I found out her married name. We saw each other (just us two) for about 3 years and then she once again suddenly cut me off. Later I would find out there was something tramatic that happened to her and that is why.
Anyway about 5 years ago, fate placed me and her BM in the same jury pool and we discussed everything. BM relayed to my sister some things we discussed and within 1 week my sister contacted me again. We have been in contact ever since and went to dinner together last 2 weeks ago. She started talking to my brother again via FB only about a year ago. We have had a strained relationships because of the adults in our situation. She is my sister though and when my brother (also her half-) passed on Thanksgiving, I had to go back up to the hospital with her after the rest of the family left so she could say goodbye to my brother who was on life-support pending a donor transplant. Not because my family would have cared one bit that she came there but because her BM pulled so much PA against her as a child that she feels out of place in our family. Her BM of course is nice as can be to me and we are FB friends but both she and my dad are to blame for this.
My point is her BM and my father (because he probably could have done a little more than he did) caused us to get to this point....My sister and I have so much in common. We talk forever when we are together. She is my sister even though we weren't raised together. She loved my brother and didn't even feel like she could come to his funeral and only got to say goodbye at the hospital.
I beg you to try to foster a relationship between your child and their sibling. As a person who would do anything for my (half-)siblings and feels robbed of time with her oldest sister.