Is it permissible to marry one's paternal aunt's daughter if her elder brother is a milk-sibling? And is it permissible to marry the daughter of one's father's maternal uncle?

There is no objection to your marrying either of the two girls. The fact that one of their brothers was suckled with you does not extend to her sisters, unless you were suckled by the girl's mother. The second girl is not forbidden to you simply because her father is your father's maternal uncle. You may refer to Fatwa no. 151865 for more details on forbidden women.