Is refusing to sit next to one's sister in the car to avoid touching parts of the body that are unlawful to look at considered extremism or a correct action?

It is permissible for a brother to sit close to his sister, as long as she is dressed and there is no lust between them. Scholars have permitted a mahram to carry his mahram relatives during travel, and what ensues from that, such as touching the body through clothes when there is no lust. This is because the awrah (private parts) are covered by clothes, and lust is absent due to the mahramiyyah (unmarriageable kinship). This is what Al-Sarakhsi mentioned in Al-Mabsut. Therefore, refraining from sitting next to one's sister might be an act of excessive strictness.