Hatching time

Hi everyone,
This is my first post and hope someone can help. I bought 3 bantam wyandottes at POL. All was well for a few months, but... Dot, the first to lay became broody, followed a week later by Tilly, then Vera followed. I got Dot 4 eggs from ebay. She is on day 20 and stillno sign. I have candles them a few times and not sure they are viable. When Tilly went broody I put two fertilised eggs from a friends hens under her and they are developing well at day 11. Tilly and vera are also sharing six hatching eggs too and wont be separated. So, my question is can I  take the 11 day eggs and put them under  Dot, who has been brooding for just over 3 weeks altogether? She is eating, drinking and dust bathing every day. Could she go another 10 days?