Sons of Svanir in Hoelbrak

What are they doing in Hoelbrak? And most importantly, why their presence here is tolerated by Norns?

They are allowed for the same reason the Inquest is allowed in Rata Sum, so long as they aren't actively trying to harm people in the city, most don't care.

The Norn don't judge people by the groups that they associate with, but by their own actions. So as long as the Sons of Svanir don't do anything worthy of punishment or death, they are allowed.

Narcemus already nailed it, but I think one of the original sources did elaborate on things quite well:

"Norn culture stresses individuality. It demands that a person be judged by their own actions, not by the actions of a group to which they belong. If three Sons of Svanir attack a shrine, those individuals are hunted down and punished. That does not mean another norn who claims to be part of the Sons of Svanir will be punished or treated badly because of the event—in the norn mindset, he didn't do it, so he isn't to be blamed. This doesn't mean that the norn ignore a person's allegiances or that they don't understand Sons of Svanir are dangerous people. It is simply that, as a race, norn do not judge an individual for the sins of his tribe.

A norn lives and dies by her own legend."

If memory serves me right, I think that very question can be asked ingame to nearby NPCs somewhere in Hoelbrak, with the same answer that individuals are judged by their actions. They are unpopular, but as long as they don't cause (much) trouble, everything is good.

On a sidenote, Braham blames all Elder Dragons for the death of his mother and not just Mordremoth. I remain convinced that he not a Norn at all.

Good answers all around in here.

There is also a very complicated (some would say contradictionary) relationship between the Norns and Jormag: while they seemingly consider him a source of great evil done to them (and others) they also see Dragon as a "proper" Spirit of the Wild that you're allowed to worship like any other - this is most likely why the Sons are tolerated by other Norns.

The Sons are also much less antagonistic towards the "good guys" than let's say the Flame Legion. While the mobs you encounter are aggressive and hostile towards the player, the faction itself does not wage outright war against the main faction in the same way that, for example, the Flame Legion does towards the other Charr or directly oppose their ideology in the same way the Nightmare Court or Inquest does. Instead they focus on converting others to their cause willingly - although with the increase in Jormag activity all around, this has become more and more "by force" rather than willingly as of late (which is why the Sons are drawing the ire of the other Norns more and more as you progress your Norn's personal story).