Need advice!

My fiancee's ex-girlfriend, who just so happens to be partner in a family law firm, is currently the alternate trustee on his life insurance policy. His mother is the trustee. I understand that their platonic friendship turned into an intimate one and they eventually became lovers. She was someone he could trust. Although their relationship transpired before I met my fiancée, I feel as though the time has come to rethink some things and remove her name from any legal documents. He has two beautiful daughters (who are equal beneficiaries) of which I help to care for so if something happens to him, shouldn't I (his future wife) be the trustee?
I asked him about the arrangements a few months ago and he turned around and asked me whether I would be able to argue/fight in court if his ex-wife tried to intervene and take measures to secure the life insurance money that he has set aside for his daughters.
I know she is a lawyer and could probably do a better job than me (in arguing his case, that is) but I feel if he trusts me enough to marry me entrusts me with the love and care of his daughters then I should be made trustee. It is not about the money. I really don't care about that. It just don't seem ethical to have my future husband's ex-girlfiend/mistress as the trustee on his life insurance policy as if I'm not capable.
Am I just being paranoid or do I have a vaild point? In either case, I don't know what to do? Please help!!

Is this ex wife the mother of those children? That would be the one question I'd have to ask myself. It it were me, I would have the mother of the children be the trustee for the children's share, and change the mom one to you. It's not about trusting you, it's about making sure his kids are taken care of I think.