Haruiro Astronaut manga

Mami and Tomoki have always shared everything in life, they are
identical twins, yet so different in character and personality:
Mami tender and helpless, courted by boys to whom she doesn't never
"no" for excessive kindness, Tomoki with a strong sense of duty and
a great spirit of sacrifice, would do anything to see her sister
happy. But comes a time, however, when one realizes there are
things cannot be shared, especially when it is something extremely
rare and unique .... just like an alien?!