Probably sisters and junior members of the family of Utenhove. Hendrik van Utenhove (d. 1715) bought Bottestein in 1703, and the house was in the family's possession until 1855.
Likely candidates, according to my research, are Elisabeth Wilhelmina van Utenhove (1744-1796) and Charlotte Martha van Utenhove (1743-1788), both daughters of Jacob van Utenhove (1700-1767) and Geertruid Scheepers (1718-1753). Their brother Maurits Carel van Utenhove (1747-1780) was known as Heer van Bottestein at his death. Their father, Jacob, was a grandson of the Hendrik van Utenhove who bought the house in 1703.
Life with Boswell:
Boswell went to at least two assemblies at the Mademoiselles Bottestein, but does not appear to have described them in any details. (not in his published papers, at least)