Arwen sits pretty sloppy unless there is a treat nearby or we are working. She's always been that way so I never thought much of it, her hips X-rayed good in prelimb so it boils down to her just being a lazy lump.
If your girl was spayed recently it could be that when she did sit straight it hurt, so she got use to sitting "lazy" while she was healing and she is still just gun-shy of trying to sit up straight again. If she has been okayed by a vet maybe try straightning her out with a treat and praising only when she is sitting "pretty". They really can train themselves to be sloppy much faster then we train them to be pretty.