When & how do I tell my daughter about her half siblings?

I have a complicated situation and need some advice. I have been married to my husband for 8 years, we have a daughter, age 4. My husband was married previously and had two children. When my husband and his ex divorced it was crazy ugly and the kids were being torn apart by the fighting. Long story short, my husband ended up voluntarily relinquishing his parental rights so the kids could be adopted by his exs new husband. The kids were 3 & 4 at the time, and had been with their mom and step-dad almost exclusively for about a year, only seeing my husband and I every other weekend. My husband did what he did with the agreement between him and his ex that he would still have contact with the kids several times a year. As soon as the adoption was finalized she went back on her word and cut off all contact. The kids are currently 12 and 13. They know about my daughter and have said they have no interest in any relationship with her. My husbandҒs parents have been allowed to stay in the kids lives throughout the years. My husbandҒs parents are also very close with his ex and her husband, supposedly just to be able to keep seeing the kidsӔ. Our relationship with his parents has been full of tension and arguments since my daughter was born due to their lack of interest in her, all while doting on the other kids. We live 6 hours away from his parents so we see them only a couple times a year. They do not call our daughter and do not send cards or gifts on holidays. The gist of our relationship is me posting pics of our daughter on facebook for his parents to see. I do this because I keep up the hope that one day they will finally show interest in her and want a relationship. They are the only grandparents my daughter has so I cant bring myself to completely give up even though they donҒt try at all. When we do visit them at their home, pictures of our daughter are laying around on shelves while pictures of the other kids are framed and displayed. They also talk about them A LOT. Ok, now that you have general background, my dilemma is this: When and how do I tell my daughter about her half siblings? On one hand Im happy with her knowing nothing and preserving the ғnormal family she currently has, but on the other hand, I donԒt want to lie to her, plus her grandparents relationship with the other kids makes it impossible to hide forever anyway. My fear is her being hurt. I am certain that at whatever age she is when we tell her about her siblings she is going to want to meet them and will be hurt when she learns they donҒt want that. I know right now she is too young to even come close to understanding the situation but I also dont want to drop a huge big bombshell on her and have her feel that sheҒs been lied to all her life. I know the situation is convoluted and rare, I suppose, but Im here hoping to gain some good advice. Thanks for listening to my long tale!