Danielle - who has children Mimi, three, and five-month-old Hendrix with fiance George Lee - said: "I'm so sick of pretending everything's OK. I don't even know if she's aware I've recently had a baby. Mel hasn't spoken to any of her family or friends in the UK for over two years.
"I don't have her number, I don't know where she lives - I wouldn't know how to get in touch with her. I follow her on twitter to make sure she's OK."
Danielle - who still keeps in touch with her sister's former bandmate Emma Bunton - revealed the pair fell out because she doesn't like Mel's husband Stephen but hopes they can put their differences behind them because she misses her nieces.
She told Britain's OK! magazine: "I hope I will speak to Melanie again because she's my only sister. I miss her and my nieces ridiculously. It hurts."