Are the husband of the mother's maternal aunt and her children considered mahrams, making it permissible to shake hands with them and remove the hijab in front of them, given that the mother's maternal aunt breastfed the mother?

If your mother was breastfed by her sister five full feedings during the first two years of her life, then her sister becomes your grandmother through suckling (rada’a), and her husband becomes your grandfather, and her children become your maternal uncles through suckling. Thus, it is permissible for you to shake hands with them and uncover your face in front of them. The proof for this is the hadith (definition): "What is prohibited by lineage is prohibited by suckling." The consideration of five feedings is based on what was narrated by Aisha, may Allah be pleased with her. If this condition is not met, then no prohibition is established.