Board Thread:Lore Discussion/@comment-3539474-20130910213054/@comment-25109952-20130921221902

Well, let's look at how it could happen.

Harkon would not become the Jarl until his father/mother died. He could marry and have children easily.

Riften was known for being the shadiest city in Skyrim.

Even a Jarl's son was above everybody except the Jarl, therefore he could still be a 'king' of sorts.

The Jarl could have been on a visit to another city while Harkon was gathering innocents; a few well placed bribes among the guards would have ensured loyalty.