Thank you for your question. I am sorry for the delay caused that at this point I am the only active European law expert, and I do have a main job as an attorney.
To answer your questions:
1. Yes, according to Article 206 and related of the French Civil Code, as a daughter in law, are obliged to support your mother in law, regardless of whether your husband has or does not have assets. This obligation ends / does not exist only when either of the following takes / has taken place:
a) your husband passes, together with any common children you may have, which of course I hope does not happen;
b) you divorce;
c) if your mother in law gravely disrespected her obligations to you. This translates as if you were ever in need, obtained a court order towards her, and she did not pay, which I assume is not the case.
2. Strictly legally speaking, you cannot protect your assets from her. As long as the assets are in your, or your husband's name, they will be taken into account when setting the amount of support she receives, and they can be seized. There is no upper legal limit. The judge decides according to her need and your global assets and revenues. Of course, you may make representations, particularly as to:
a. already existing debts, which are always taken into account by diminishing the net value of your assets in the court's calculation;
b. future obligations of maintenance, such as minor children, whether your husband is ill or unable to work, and you support him, and other such obligations (proven) towards third parties.
3. There are two means to protect yourself. One comes naturally, the other is more complicated.
A. The "natural" means of protection is that, if there is no agreement, the mother in law (or a family member designated as her legal guardian) must go to court against you specifically for this financial support. This means they have to manage to summon you wherever you are (which is not quite easy on a sailboat), then there is the entire process to go through, and then enforcement, particularly if your assets are in NZ and they are in France (normally, there would have to be a second trial in NZ for the recognition of the financial award, plus costs for the bailiff in NZ). In other words, this gives you time, although cannot protect you from the final effect, which is payment;
B. The more complicated means is that of course you can always think of Asset Protection Trusts and other such legal vehicles, whereby your assets (or only your money) become legally not yours anymore, but belong to trustees / financial institutions etc. At this point in time, the Cook Islands are world renowned for these, and they have a better reputation because they have been designed not to guard against tax authorities / money laundering investigations etc. (i.e. they are 'transparent' for state authorities) but only against relatives, and other private persons. I cannot tell you to use or not use them because they have their pitfalls as with any such legal vehicle (one obvious one is that you legally lose control over these assets, which become administered by the trustees).
4. Finally, you should also know that not paying the financial support, once ordered by the court, is also a criminal offence in France, resulting in imprisonment of maximum 2 years, and a fine of a maximum of EUR 15.000,-, under the heading "family abandonment". While this appears on the face of it (i.e. before any potential aggravating circumstances) as a non-extraditable offence (i.e. too small for France to ask for extradition) it could cause complications if you reach that point and ever wish to go to France.
5. On the contrary, if you do a little financial preparation, putting together all the debts you have / obligations to others, and convincing the French court that your assets are in fact of little value, you may have the financial support lowered considerably.
I hope my answer was useful and look forward to your rating, which is essential to my activity.
Dr I L Vlad