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.

What type of war are they involved in?

There's a property in the casus bellis definitions (\common\cb_types\00_cb_types.txt) can_ask_to_join_war = no that appears to govern if others can ask to join the war (other definitions disables the participants/attacker/defender ability to call allies).

The standard claim war has this set, so it does not appear intended that you can join in on your own. And since he probably cannot call you in as an ally because the two of you do not share the same liege (you are independent, he is not), you're stuck with hoping he spends your cash.

Alternately, try and find another CB on the hostile duke and kill his armies.