assuming u're playing vanilla (=normal LoD) let me suggest the following:
In order for your assassin to be hell viable you need to make sure to focus on 1 of her trees perhaps with a slight investment into another but not more.
A viable option would be to focus on traps (bartuc's will be quite acceptable, but look for magic +3 traps claws with nice extraskills like lightning sentry etc...) with spare skills going into shadow warrior/master for a meatshield and 1 point into shuriken (that thing where she launches shurikens with a manacost of 3-4 per bolt). This way you've got it all covered (lightning, fire (from the explosion of death sentry) and physical from that aforementioned skill.
For your brother I would recommend either a fury druid with maxed oaksage or heart of the wolverine + a single point into grizzly (perhaps better to find this as an oskill on a magic/rare druid helmet though) OR a paladin with Zeal/Smite/Vengeance and either Conviction aura (to help your traps and his vengeance) or Fanaticism (if he prefers smite/Zeal).
Those pairs should be quite viable although a barb can contribute quite well also (his shouts) and with a massive weapon is a real killing machine also.
your choice
Does it happen anytime or under some certain conditions? (doing quest, or too high skill)