Hmm, the revenge bit might be a bit optional. Here is an idea, not yet really refined:
Say, after the player discovers the betrayal, the bastards splits off for a bit. Might only be a different inn while the party is in town, or similar. Then (s)he gets abducted while away from the party, and threatened with .... whatever we think might be appropriate

(this might be where we differ

) The player gets the news the bastard is abducted, but also that there is an opportunity to get a weapon of choice or so, but only if moving now. The weapon might be on a ship, for example, ready to cast off. Which gives the player the choice: Rescuing the bastard, with the reward of gloating a bit and keeping a useful party member;
or getting the sword, with the obvious reward of having the sword, and additionally getting revenge. (Either the bastard is dead, if I have my way

- or (s)he is released by the captor but miffed and breaks completely with the party.)
In case the game does not enter the situation, you hear about the bonus weapon, but are not at the right time at the right place. Just to avoid punishing the player for choosing an interesting path through the game.
Oh, and while un-romanceable chars would be "safe", I think having a romanceable char as bastard would make much more sense, and give much more player involvement. Just make sure the player is warned.
Edit: On reflection, it does not feel right; better would be: First a choice to get rid of the bastard (either with boot on the backside or with dagger in various anatomical parts

) - if yes, player gets sword. If no, choose rescue or sword. Gives the player the opportunity to do it to the bastard herself/himself.