Am I obligated to attend congregational prayers at one of the brothers' homes, knowing that it is not a mosque, and would I be sinful for not attending?

Scholars differed concerning the ruling on congregational prayer (Salat al-Jama'ah): The Hanafis and Malikis consider it a confirmed Sunnah (definition), while the Shafi'is consider it a communal obligation (Fard Kifayah). As for the Hanbalis, they consider it an individual obligation (Fard 'Ain) upon those for whom Friday prayer is obligatory, and they do not stipulate the mosque for it. The Dhahiris consider it a condition for the validity of the prayer and stipulate the mosque for it. According to the view of the majority, the mosque is not stipulated for establishing the congregation. However, according to the opinion of the Dhahiris, the absence of a mosque is considered an excuse, while emphasizing the encouragement to build a mosque.