Jamieson, Fausset and Brown on Acts 12:12-17

12-17. he came to the house of Mary, &c.--who "must have had a house of some pretensions to receive a large number; and, accordingly, we read that her brother Barnabas (Col 4:10) was a person of substance (Ac 4:37). She must also have been distinguished for faith and courage to allow such a meeting in the face of persecution" [Webster and Wilkinson]. To such a house it was natural that Peter should come.

mother of John ... Mark--so called to distinguish him from the apostle of that name, and to distinguish her from the other Marys.

where many were gathered together praying--doubtless for Peter's deliverance, and continuing, no doubt, on this the last of the days of unleavened bread, which was their last hope, all night in prayer to God.