What is the ruling on a woman riding behind her sister's husband on a motorcycle, with her legs open, clinging to him, and holding his sides?

The sister's husband is not a mahram. Therefore, it is impermissible for the wife's sister to ride behind him in such a close and touching manner, as this is a clear abomination and a gateway to temptation and corruption. This is prohibited to prevent pretexts and to guard against forbidden desires.