Purchase Link (DLSite): https://www.dlsite.com/maniax/work/=/product_id/RJ272987.html Purchase Link (DMM/FANZA): https://www.dmm.co.jp/dc/doujin/-/detail/=/cid=d_168452/
They're twins and they both swap genders (not body swap) when they woke up, so I added the female:genderbender tag. They're identical, so it's almost as if nothing changed, but you can tell based on the eye and glasses colors, and their outfits from the first page.
Someone asked for a synopsis elsewhere, so I'll copy-paste mine:
Title is: "Our genders flipped and big brother gave me a tit job"
After many years away her big bro comes back home. She doesn't know how she feels about him since it's been so long. They never talked much and things are super awkward now. She thinks it'd be easier to talk to him if he was a big sister instead. Next morning their genders are flipped she can't help staring at his huge tits, gets boner, hides in bathroom. The big "sister" barges in and says "if you're gonna jerk off, let me do it. She wonders "is this big bro's secret side?"