“I do not mean to suggest that the birth of a sibling will always qualify as a material change in circumstances. Whether the birth of a sibling constitutes a material change is contextual, and can only be determined by examining the specific facts of each case and the precise terms of the parenting schedule sought to be varied: MacDonald v. MacDonald, 2014 ABCA 91, at paras. 6 and 12; Simmons v. Simmons, 2016 ABQB 479, at para. 37; Garland v. Brouwer, 2016 ONSC 5966, at para. 7.”