Although you and I are among the very few people who talk about lesbian relationships, I don't think Haanja and Lelia have a romantic connection (but I wish they did lol, c'mon, our two nuns need love). I see it more as an anime cliché where the flat-bodied girl, without breasts or butt, is the best friend of a hot girl. Haanja even commented in the audio about Lelia's breasts being large and hers being quite small, which I find funny...
But their relationship is very well detailed in the audios. Haanja has a lot of respect and admiration for Lelia, she has no doubts about her ability and strength. In the audio with the demihumans, when she saw Lelia dead, Haanja refused to believe she had died, and when she was being raped, she lamented being weak and leaving Lelia to fight alone, showing the feeling that she could have done more for her friend. Similarly, in the audio where she was sucking milk from Lelia's breasts, she said she was full of milk inside but it was worth it for having helped. Lelia, and if she could, she would do so much more for she...
I don't know if the creator has revealed Lelia and Haanja's ages, but Haanja's calmness and tranquility are incredible. Sometimes she seems calmer and colder than the protagonist herself. Haanja manages to calm and reassure Lelia with just words, showing no fear or apprehension about what she's going to do to help her, like sucking Lelia's breasts or pseudopenis. And even though the ASMR only shows Haanja's POV, I highlight how Haanja only needs to speak once in the audio to convince Lelia to do what's being asked, showing that Lelia completely trusts her body and soul in Haanja...
Overall, Lelia and Haanja have complete admiration and trust for each other. At no point in the game do we see them having problems with each other, such as lack of trust or differing ideas. It's not shown what they did before the events of the game, but probably because they are exorcists, they met in a convent when they were younger and became best friends. This friendship goes far beyond the convent; outside of it, they must talk about more personal and intimate things, as best friends usually do. They probably consider themselves sisters, not in the sense of being nuns, but in the sense of truly considering each other as sisters.