Although Selena Marie Gomez and Rebbeca Marie Gomez have the same middle and last names- they are not whatsoever related as stated by Becky herself. However, it is possible that they have related ancestors.
They aren't related because they don't come from the same bloodline. There are a lot of people with the last name of Gomez, because it is a common last name, so not everyone with the last name of Gomez is related.
They are not sisters and are in no way related (as far as either of them know- they could be related but it would be very far back.) Becky's parents are Francisco and Alejandra Gomez. Selena's parents are Mandy Teefey and Ricardo Gomez.