Just to update the community and this information is very useful as it is at variance with what we from the UK would expect or assume, it is French law that sons and daughters in law owe a duty to a parent in law in need of sustenance (basically any living costs unless you let them live with you) and **that does not expire on death of your spouse if you have surviving children. This is contained in Article 206 of the French Code Civil. I have copied the translation below. It would also seem that just because your in laws have their own surviving children this obligation falls in no particular order. It does not end if you get remarried either.
It is however reciprocal therefore if you find yourself in need you can request that your in laws assist you and they are also bound to do so. Also your duty to yourself and your close kin takes precedence and if you cannot afford to contribute that is a plea you need to make under article 208.
It seems a fairly onerous obligation but having researched it as with many of these laws it has historical roots that make sense and possibly our culture differs a lot in this respect. Certainly in Scotland the in-laws are usually suffered or feared or both. To have to accept a lifetime bond at marriage would probably end the institution of marriage in Scotland. I spent sometime looking at the Code Civil in respect of families and it does instil a duty on families to look after each other which differs from the UK situation and I would suggest if you live in France you do get to know how you are affected by the obligations of the civil code.
If you live in France you fall under the Code Civil.
If any one has questions on this please contact me.
Art. 205
(Act n° 72-3 of 3 Jan. 1972)
Children owe maintenance to their father and mother or other ascendants who are in need.
Art. 206
(Act of 9 Aug. 1919)
Sons- and daughters-in-law owe likewise and under the same circumstances, maintenance to their father- and mother-in-law, but this obligation ceases where the spouse owing to whom the affinity existed and the children born of his or her union with the other spouse are dead.