Hokay, so I believe the older sister is Minami and younger is Akina. Either way I'll go with those names.
Basically: Akina starts out telling her BF she and her sister have the ability to swap bodies. So first swap is she finds her older sister sleeping and swaps with her. After she leaves the room you see Akina come back and she has sex with him using her "real body" at which point the older sister enters the room again shouting "What are you doing with Keita, Minami-oneechan!" to which we find out that she had woken up in Akina's body and pretended to be her and had sex with him as punishment for taking her body without asking. Afterwards Minami says she'll swap back with Akina but then she wants to try something. That something is swapping with Keita and fucking her own body with Keita in it.
Next she puts Keita in Akina's body and scissors with her commenting on the rest of Akina's punishment is sending her back to Keita's home and he gets to do whatever he likes with her body until tomorrow. He comments on her body being too amazing and sensitive and stayed up until 2am masturbating. In the morning Minami has a cold and stays home from school but says wait one minute to Keita. It flips to school then and its apparent she swapped with Keita to leave him in her body at home. She pretends to be Keita and "swaps back" with Akina so she's in her own body, but she thinks its Keita back in his own body when its really Minami. Keita then masturbates all day as Minami despite being sick. The end.