宜 is a compound ideograph. The oracle bone script of 宜 depicts two pieces of meat on a sacrificial vessel. The line in the middle of the two pieces of meat could mean “divide or share equally”. In the bronze inscription, one line was changed into two. In the seal script, the meat was written as a 月 and the sacrificial vessel was written as 宀.
The sacrifice is a delicacy, so the meaning of 宜 was extended to:
1. adj. suitable, right, fitting, proper
2. v. should, ought to
3. adv. certainly
See also:
Radical meat 月: on the left or right?