I'd focus on the Sword and Shield, Champion and Vanguard trees (though Templar works surprisingly well too, if you are looking to deal some damage too). The key of keeping a warrior alive is constatlny generating/maintaining guard, as this adds an extra "health bar" to their already large health pool.
Sword and Shield tree:
Shield Wall and Shield Wall and Shield Wall. This is hands down the best ability for tank characters, as it blocks/reduces damage taken and builds guard in the process. Immovable object upgrade is recommended if you have Trespasser installed. Make sure to set it to preferred under tactics.
The next ability you should definitely take is Bear mauls the wolves, as you'll be protected from all angles and enemies cannot backstab you. Warrior's Resolve, Turn the blade and Turn the bolt also help increase durability with the incoming attack damage reduction, consitution bonus and stamina recovery (which helps you keep up shield wall longer).
Virtually every passive ability from this tree really useful in a tank build. The active abilites like Shield bash etc. are optional, if you are not playing your inquisitor mainly I guess you will be fine by having her on full defense, so she does not really needs advanced offensive abilites.
Vanguard tree:
Where Sword and Shield tree increased the warriors durability, the Vanguard tree boost the other important skill of a good tank - and that is threat generation, as you'd want the enemies to focus on the most durable member of the party. Your first and best bet for threat generation (and guard generation too) is War Cry, as it aggroes every enemy around you and builds oyur guard based on how many enemies surround you. I'd recommend the Call to Arms upgrade, as it gives you a temporary 200% armor bonus too.
The Challenge ability is similar, but in my experience it is only worth it with the Call them out upgrade, as it removes the cooldown and you can spam it easily (it costs silightly less stamina than War cry) but it builds less guard, so I think you are better off with War cry and use the points on other abilites. If you are surrounded by a large number of opponents, Livid goes well with War Cry, as it does not consume stamina and builds guard.
Again, any passives from this tree help you a lot in increasing your durability and threat generation. The Bodyguard ability I find useless, as if you are doing a good job as a tank, you won't need to take the blow for anyone in your party.
EITHER
Champion tree:
Line in the Sand is mandatory, as it is needed to acces the rest of the Champion abilites, but its use is really situational, as it works better in tight spaces (inside caves or builinds), but in wide open spaces, you could use your mage companion to build some ice walls and create a single coridoor for your warrior to utilize Line in the sand. Everlasting Barricade upgrade recommended. I wouldn't recommend To the death, there is no need totake the risk by giving your enemies increasing damage output, with the right utilization of tactics, postioning and builds, anything can be beaten conventionally under a reasonable ammout of time. Walking fortress is great (Siege Breaker upgrade recommended), it can be a real lifesaver if you are in a tight situation, or if you want to feel like a bada$$ while clearing out enemies. Any and all passives come highly recommended, as usual. The focus ability, Counterstrike can prove useful if threat management slips out of your hands, as it automatically directs everyone's attention to the Champion.
OR
Templar tree:
Templar can be a viable alternative to Champion, it focuses more on dealing some damage, but it is great against mages and demons (which you'd be facing a lot in this game). Imo every ablitiy is really useful from this tree, as they have reasonably low cooldowns and provide great benefits, plus the abilites in this tree are pretty straightforward too.