I have a joint banking account, will I earn points when the sister cards make transactions?

Yes, you will earn points when any of the cards you have linked through online banking credentials when you make purchases at your activated offers. You only have to link one of the joint banking account cards to earn points (in fact, Drop won't let you link both!)

To check which cards are linked, open the Drop app, tap onto Profile (top left) > Linked Cards, and choose your bank.