Please do feel for a penis ridge and look inside the slit. A boar slit goes straight down, a sow slit has a fleshy seal just inside. Compare this one in the picture to the other one you know is female - does it look different?
It may help if you post pictures of both piggies genitals.
Sexing needs to be a hands on thing, and its never safe for us to sex a piggy via a picture as we cannot see inside the slit in this picture nor can we feel for a penis ridge, I do have my thoughts and I would not be too certain this is female but only you can do the physical checks