Come on, Iolo, you know exactly what Gwyneth is up to, so all you have to do is set your mobile phone to “voice recording” or whatever, stick it in in your top pocket and engage her in conversation, while Siôn is out of the way.
Tell her, “I know exactly what you’re doing – you’re simply after Dad’s money and the house!” and if it works like it did the last time, that should get her admitting what she is doing, for later playback to Siôn.