Because in a World of Action Girls where the roles of men are often trivialized (both in-universe and in-story), how else are you going to have Shipping? Oh, and the copious ecchi helps.
In the first episode, Elina is clearly disappointed that she doesn't get to sleep with Reina.
In the first Utsukishiki Toushi-Tachi OVA, Elina reminisces fondly on how, in the innocence of childhood, she was able to sleep and bathe with Reina at will, and the episode focuses in part on her sadness that, as Reina aged, she began to see such things as improper, while Elina still desires to do them... in a less innocent way.
Another OVA, setting things in a High School AU, had one of the subplots focus on Elina's attempts to take advantage of her sister, and threw Claudette into the mix as well.
Reina generally serves as an Even the Girls Want Her for many of the women she meets, outside of the explicity-hetero Cattleya and Nanael. Further examples:
Echidna, who administers an antidote mouth-to-mouth. Or we can just strike it and say the entire oil-fight between the two of them was one big steaming pile of Les Yay.
It's implied that Echidna molested Reina on several occasions throughout the first season of the first series.
Her relationship with Tomoe seems platonic enough, but that doesn't stop Elina from getting jealous.
Reina herself seems to have somewhat of a crush on Listy, as expressed by how she follows her around in the first season, or later how she is desperate to win her approval, or in the second season when she must rescue Listy from Aldra's Mind Control.
Echidna has tiny implications of bisexuality, but at least definitely lesbian.
In her backstory with Irma, it is shown that they slept together while she was training her, in something resembling pederasty.
The aforementioned oil fight with Reina, and her molestation of her.
Basically her end story with Irma implies that she settles down Happily Ever After with her, briefly appearing in the first Beautiful Fighters OVA to fight Elina, but later saying "I have some cats to take care of." Irma is a Kindhearted Cat Lover.
Echidna's interactions with fellow elf Alleyne sees her teasing the combat instructor like long-departed lovers.
Menace is another explicit lesbian, with her backstory detailing betrayal by her lover in the ancient kingdom of Amara which led to her overthrow and death.
In keeping with the above, Menace spends the entire series trying to Mind Control various individuals, preferring women. At one point she placed Reina under this control.
In the Beautiful Fighters OVAs, she has built a kingdom of willing servants (all masochists). Her handmaidens are scantily clad, and at one point she drinks wine using their cleavage as a cup.
Airi, who regenerates her power by deep-kissing people to suck the life out of them. She seems to prefer female victims.
Tomoe and Shizuka often seem awfully buddy-buddy. Heterosexual Life Partners? Or something more? You decide.
Aldra seems to have fun with taking over Listy's mind, putting her in an Absolute Cleavage catsuit in the process. Rebellion gives her a husband (completely off-screen, no less), but her Les Yay tendencies remain.
Melpha, in turn, seems rather taken with the angel Nanael.
Queen's Blade Rebellion
The anime of Rebellion might as well be called There's Something About Annelotte. She's surrounded by girls who are, to varying degrees, taken with her, which she seems to take in stride.
There's Yuit, who knew Annelotte growing up, but Annelotte was raised as a male, so Yuit still calls her Onii-chan, much to Annelotte's chagrin, and is just as competitive for her attention and affection as the others in their makeshift party.
Mirim also seems to look up to Annelotte as a bigger sister figure (Oneechan), but that entails bathing with her and sleeping with her, and competing for the privelege.
Lunaluna is an oddly mixed case, since she perceives Annelotte to be her "Destined One," and was, at first, visibly disappointed to find out that her chosen one was a woman (as she starts by asking Annelotte how many children she wants). But then Lunaluna's "moon side" seems as hungrily perverse as Echidna.
Sanyang, who sees herself as having a life-debt to Annelotte, and once again, is eager to compete for affection from her.
Then there's all the severe hinting that Annelotte is Aldra's long-lost sister. When it seems like Annelotte's dark side is getting out of control, we're presented with a Kannazuki no Miko -esque naked embrace scene, albeit as an Imagine Spot or something similar.
