@Naif1 Did you think the whole flat was for you and your sister because you failed to fully read the listing description, or because the host misrepresented it?
If it was written that you would be renting an entire dwelling, so misrepresented, then you would be entitled to a refund and you would have to contact Airbnb about that.
If you simply didn't bother to read comprehensively about what you were renting, than you'll have to abide by the cancellation policy you agreed to when you booked.
When you booked the listing, you would have gone in and searched either for a whole listing or accommodation shared with a host or other guests.
Go back to your booking confirmation and check to see what you have booked - what does it say?
If it says you booked a place to yourselves, then you can contact Airbnb and ask them to cancel the booking as the listing is not as described. If not, it seems that you may not have read the description properly if it mentions it is a shared place and that you will share with other guests