I've looked into it and from my research cockatiels seem not to be great foster parents of other species. However, a friend of mine recently fostered a Ringneck baby to a pair of cockatiels and it worked, though she had to remove the baby when his beak got bigger than the cockatiel beaks.
But if you move forward with it, I would say you need to be prepared with the right equipment, knowledge, and determination to raise a day 1 baby. I'm not familiar with splendid parakeets but in cockatiels and similar sized birds a day 1 baby has to be fed every hour for multiple days, then every two hours, and so on and so forth.
How long has it been since the egg was laid? Some birds don't start sitting until the last egg is laid.
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Cockatiels:Battle, Sniper, Jackie, Honi
Blue Headed Pionus: Mel
Green Cheeked Conures: Flick, Cache, Titan
Indian Ringeck Parakeets: Blue, Kamehameha, Ka'iulani, Kahili, Shadow, Ali'i, Noni, Pono, Smokey, Hapu'u