Maddy was 2 yeasrs old when I met my now DH & he has never ever called her his step daughter (we dont have another that's only 4 mths age difference either though).
I think I'd call her your daughter too, no need to explain as JohT said..

I think I
d be hurt if I heard Dh say "Oh this is maddy & This is Indah, but Maddy isnt mine!"

She lives with you fulltime calls you Mum & you obviously love her like your own, just because she was not birthed by you, does not make her any less your daughter!?

Or I'd simply say "Yes they are twins"... Why not? who cares what strangers say or think???
Ask the girls, maybe they'd love to be known as twins!!!