@dreminh It's not. I think there was a chapter or two where Mahiro got their comrade back. The sister should have an "Antidote" or some other way to turn Mahiro back, and soaking in that medicinal hotspring apparently was a cure of its own, so it's not too far to say it could be cured on its own even without the antidote.
I bought the stickers when I first saw it on the pixiv page, would really want to support the author by buying their books physically but shipping and stuff from japan is confusing...
@Apryl
well whatsapp isn't popular in Japan since it needs phone number and for Japanese it's a really private things (only shared to close friends/family) so I don't think they will release something for whatsapp
you can rip it and make your own whatsapp sticker though, if you want it
@ravidya Over time, Mahiro will turn back into a guy. There's no need for an 'antidote' as there's a time limit to the effect. It just happened to be that the effect was starting to wane during their trip to the hot spring. But her sister was prepared and had the medicine to turn Mahiro back into a girl.