How to become a king of a friendly dynastic kingdom?
Solution 1:
This one depends on the specifics on the situation, but:
If (1) Chernigov is larger than your kingdom and (2) You are christian, you could ask the Pope for an invasion CB. If Chernigov is losing a war, you might be able to swoop in and claim a quick victory.
If Chernigov is just one or two counties bigger, you could grant independence to a couple of counties so that you are smaller, take Chernigov, then reconquer the counties spun-off later.