First I must say that we do not give medical advice on this SO, I am not a doctor and am not providing advice of any kind. You should consult a doctor for any action you take.
There is a 2016 study where the subjects were instructed to sit on a chair, the height of which was adjustable, so that their knee and hip joints were bent at 90°.
In this study, the right leg-crossed sitting posture led to a decline in the right trunk length with time and, in terms of pelvic torsion, increased the posterior rotation of the right pelvis when compared with the left pelvis. In terms of the state of the spinal alignment, this posture generated concavity on the right side and convexity on the left side. In addition, it was analyzed that spinal deformities were caused by the posterior rotation of the right pelvis.
In individuals with a limited range of hip joint motion, this will increase the pelvic rotation. Therefore, the findings of this study suggest that crossing a leg while sitting can cause scoliosis and spinal deformities.
It referenced a 2005 study:
One previous study that had 20 healthy adult men perform a one-leg-crossed sitting posture reported statistically significant differences in the muscle activity of the musculus rectus abdominis, internal oblique, and external oblique muscles after the intervention. This resulted from the one-leg-crossed posture, causing asymmetry of the trunk muscles.
And another 1975 study:
In most cases, however, people cross a leg while sitting because this posture feels comfortable or has become a habit. This causes left-right asymmetry in the body and increases the rotation of the lumbar spine by rotating the pelvis, following the bending of the hip joint
And take note of the limitations they give at the end:
This study has some limitations regarding the generalization of its results. First, it did not observe the subjects in the one-leg-crossed sitting posture for long hours, and it involved a relatively small number of subjects. Second, it could not identify the effects of the one-leg-crossed sitting posture on trunk stabilizers. Therefore, future studies should be conducted to identify the correlation between unstable postures commonly adopted in daily life and musculoskeletal diseases.
It seems there are effects on your spine from crossing your legs while you sit, enough to potentially cause both scoliosis and asymmetry in your trunk muscles. Favoring one side could end up in more harm than good by messing up your scoliosis even worse, or it could potentially help. Again, what you should take away from this is that you should ask a doctor and evaluate from there.