Is it permissible for a brother to place his hand on his sister's chest while he is joking with her?

The jurisprudential rule is: "Whatever is unlawful to look at is unlawful to touch," because touching is more intense in pleasure. The preponderant opinion is that a man is not permitted to see of his mahārim (unmarriageable kin) anything more than what is usually apparent, such as the head, neck, and feet. Accordingly, it is not permissible for a man to touch his sister's chest or that of any of his mahārim without an interposing barrier. However, with an interposing barrier, there is no harm if temptation is absent. But joking in such matters is inappropriate, as leniency in it leads to grave consequences.