Does a state of mahramiyyah become established between the questioner and all of her paternal aunt's children—males and females—given that her paternal aunt breastfed the questioner's sister, and that the questioner's mother denies the breastfeeding extended to the elder children?

What is established by suckling is established by lineage. The suckled child becomes a son to the wet nurse and to her husband, the owner of the milk. The prohibition is specific to the suckled child alone, not to their siblings. Therefore, if your aunt suckled no one but your sister, then you and your siblings are not siblings to her children by suckling. It is obligatory to maintain ties with the aunt and to abandon the causes of estrangement between the two families.