Are the children of the maternal aunt considered siblings by Sharīʿah if she breastfed my brother, and claimed that she put the questioner and her sisters to her breast for suckling without clarifying whether milk was produced or not?

Breastfeeding is not established except by five known sucklings. The aunt's statement that she put you to her breast for breastfeeding does not prove the occurrence of breastfeeding that establishes prohibition, unless another woman testifies that she breastfed you five known sucklings.