The singer took in Zac and Magdalena in 2008, while she was with her then-husband Andy Willner.
They have since split, but Sinitta forged on with her plans to make an official application to adopt the younger sister of her two children after learning their biological mother was pregnant again.
She explains, "From that moment on, rightly or wrongly, I started to look on the baby as mine. (I felt) an overwhelming emotional attachment."
However, officials from Britain's Social Services rejected her bid to take in the baby, because she would not be able to give up work for the 18 months required of an adoptive parent.
She tells Britain's Hello! magazine, "I am very, very sad to say that, no, it does not look as though I can have this baby now. It seems that I would need to be able to not work for 18 months to meet Social Services standards and, as I have two children to support, I can't do that.
"It's been hard to pick myself up and not be depressed, because I really believed that this baby, my children and I would all be together. But I have to want what is best for the new baby, and it does bring me comfort to know the baby will go to someone who is able to stay at home and be with them, as I was with my two.
"I look at my little ones and remember how blessed I am to have them, and the lovely people they are growing to be."
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