Was your father married at the time he died? I don't know about pensions, but 401ks must go to a spouse unless the spouse signs something declining. Something like that could have happened in this case - or his wife could have signed it over to her daughter. If you want to get to the bottom of it, I think you'll have to talk to a lawyer. I'm sorry you have to deal with it at all though.
I'm estranged from my mother, and it would not be worth it to me. Actually, we had some contact a few months ago, and part was specifically about her will. I told her I wanted no part of her estate. So, I would just let it go.
That said, it's a choice only you can make. It doesn't sound like it would burn bridges, as those bridges are long gone. I wish you peace, whatever you decide.