Why can't I join my son-in-law's war?

I am a strong, independent King, who needs no man. As a consequence of my massive, throbbing brain, I have managed to matrilineally marry my female heir to a Frenchman, whom, despite his inferiour intellect, I have later orchestraed into a position as a Aquatainian super duke. It's super awesome. In theory.

Anyway, a different, weaker, duke has challenged him to depose him from perhaps his largest duchy. Since my son-in-law is stronger, this shouldn't be a problem ... except he doesn't lift a finger to prevent catostraphe from happening.

Aquiataine has been through a long period of internal strife, so the total power of these two dukes are perhaps 1,500 men each. Yet my son-in-law has let the other duke siege his provinces, and the warscore has been ticking for years.

Apart from this dispute, they are both fighting a small independence war on behalf on their common liege – this is what occupies my son-in-law.

I've sent him several hundred gold, so he now has over 500, yet he doesn't spend it on anything (I hoped for mercenaries).

Unfortunatelly, I can not join his war – that option is not available on the diplomacy screen. I assumed this is because they are not independent. My son-in-law is perfectly able to invite other parties in the same realm into the war.

Interestingly, I am able to join the opposing duke in a different war, which is also an internal strife.

I simply don't understand why I can join that war, and not the other, they seem pretty similar. I am not involved in any other relevant war that would cause me to become hostile to my allies.

Is this a bug? Is there a way I can join this war, perhaps through console commands or save-game editing? Should I just continue throwing gold after him, hoping that he will eventually hire some mercs? I am currently plotting to kill the opposing duke, but not sure I can make it in time.