Who is Betty Weller from The Cape Ann and what is their importance?

Lark's maternal aunt, Aunt Betty Weller, is hopelessly in love with Stanley Weller. During her pregnancy, she is bedridden and extremely ill. Aunt Betty delivers a stillborn baby girl who she names Marjorie. Betty then has an affair with a married man, terminates a pregnancy, and remains with Arlene and Lark for the remainder of the novel.