From: No Game No Life (Anime) ep.1 timestamp 05:47
Context: Someone sent the main characters an anonymous email and addressed them as siblings. This is their reaction:
I understand the sentence means "How did they know us BLANKs (their clan's name) are siblings?"
I also found out that 知ってんだ？ comes from 知っているのですか.
However, I would like to know how come 知ってんだ is attached to the conjunctive form of the copula （だって）. I couldn't find an answer to this anywhere. Is it even a conjunctive form of the copula? Couldn't it be a contraction for という? But in this case, it's not like it makes more sense since it's followed by another verb.