Possibly Isabella was a second marriage and the children are from the previous one. Ballaugh IGI records seem a bit hit and miss for this period and I did not find ssssssuch a marriage
Brian has a burial for a Catherine Stephen 38 Bal 26 Dec.
Also in Andreas in 1751 on HO107/2524 folio 87 p 1102 there is a Catharine E Stephen unmarried 48 Seamstress living with three sisters, one of whom, Ealy (sic), is a school mistress in Andreas. In 1841 they are together on HO107/1463 folio 45 page 7 with Catherine Stephen aged 82.