You want spoilers? I'll give you spoilers, the story is deviating a little from the LN, I can tell you what happened in the original LN to give you an idea of what the sisters are capable of, but be warned, it's bad and it's going to get worse!spoiler please
- See that scene where the MC explains to his sisters why Aina works for him? Well, in the LN, those sisters walked out in the middle of him explaining the situation.
- Those magic rings for translation? In the LN, the grandma made those rings and hid them sloppily inside their house, the idea was for the sisters to look for those rings by themselves and get the ability to speak the language in the otherworld. Well, it was too much of a bother for them, they got lazy and ordered their brother to look for the rings in their place.
This is only 2 examples that comes to my mind, but there were a lot of similar stories in the LN vol 4
And for the other spoilers
- So it turns out the egg will not hatch a Chocobo, but a loli, the sisters will want to keep her of course, but without the hassle of taking care of said loli, so of course they expect their brother to do the actual work.
- If I remember right, they will want to open a shop in the otherworld, but of course, they can't be bothered to do the work of opening the store themselves, so they will exploit their brother for investments and initial work.
And lastly
A LOT of the readers hated the sisters, how they get away with everything just because they call their brother “onii-chan” or “nii-nii”, it got so bad the author had to phase them out from the following volumes, I remember the sisters were only mentionned in 1 or 2 lines inside LN vol 5, vol 6 had no mention of them and as far as I am in the LN (volume 7 or 8 I think) they barely have any screentime, whenever there's a line about them, I'm like "oh that's right, they exists and weren't killed off".