You can find the lore of the league here.
There is also a 'newspaper' or sorts within the client (Thanks to origional poster, it is called the journal of justice I think. It is the feather just under the riots points total in upper right) but now that I think about it I have not seen an alert for a new issue in a little while. You can also find on each champions page more specific lore for that particular champion.
In specific to Sequani the journal will not provide any information it has not been updated since her release. Sequani in her lore if you read though views peace as a violation of the traditions of Freiljord (in specific to ashe's campaign for peace and joining the league). I do not have specifics for the marriage and uniting the tribes off the top of my head but given the information in her lore it is because she views the peace as a slap in the face to tradition again.