Well, it's gotta be something to note when the Japanese music business can say that it had at least two female groups with the same name of Milk. One has already been featured on "Kayo Kyoku Plus", and that was the aidoru trio including a pre-pubescent Yoko Oginome（荻野目洋子）around 1980.
However, there was also a couple of sisters who were also Milk from around the mid-1980s. Singer-songwriters Ritsuko Miyajima and Rie Miyajima（宮島律子・宮島理恵）from Saga Prefecture, the latter who would take on the name M.Rie, got together and put out at least one album "Milk" in 1987. I read in the J-Wiki article for Ritsuko (the original source being her own blog entry) that the two had been part of Naoko Kawai's（河合奈保子）backup band Natural & MILK as backing vocals and that was the inspiration for the sisters to go it alone.
One of the tracks on "Milk" is "Last dance", a pretty snazzy song by the sisters that has some urban sophistication and a feeling of some of that brassy 1960s downtown pop. Just from listening to this one alone, I'm curious as to what the rest of "Milk" sounds like and it's a pity that the sisters may not have extended their singing together to other albums. Again, according to Ritsuko's blog, "Milk" got its re-release last year in the autumn (so maybe I have a chance to acquire it!). She also mentioned that she and Rie had also backed up other singers such as Hideki Saijo（西城秀樹）and Tomoko Aran（亜蘭知子）.
On reading that the Miyajimas had been backing Kawai made me realize that perhaps often enough backup vocalists have been in understudy mode until they get their big chance to shine in the limelight. J-R&B singer Harumi Tsuyuzaki（露崎春女）had done her stint as a backup singer when Sing Like Talking was on tour, and Anzen Chitai（安全地帯）had been backing up Yosui Inoue（井上陽水）until they got their big break in the early 1980s.
M.Rie already has a couple of entries up on KKP including the adorable theme song for Pingu the Penguin. In recent years, Ritsuko has composed songs for the alphabet aidoru groups such as AKB48, NMB48 and SDN48.