Hello Michael,

Yes, you are entitled to add your mother-in-law either to your or your wife's social security. Although it would make more sense adding her to your wife's. It is important that your mother-in-law lives with you and is under your care. Remember that she must be considered as a dependant.

It is the Instituto Nacional de la Seguridad Social (INSS) who takes care of this process. Depending of where you are based it will be located in one place or another. So you must find out which is your nearest office dealing with 'tarjetas sanitarias'.

For your mother-in-law to have the right to sanitary assistance your wife will need to present the following documents:

To fill in the form which you are issued there.
Your wife's residence permit & sanitary card
Official documents showing the link between card holder and your mother-in-law.
Documentation showing that they live together and that she depends economically of her daughter. The first can be shown by means of a certificado de empadronamiento colectivo, which you can obtain at the Town Hall.

However please note that these requirements can vary from one 'comunidad autonoma' to another or even one office to another, so I would recommend that you request an appointment previously in order to ask them for the exact documents they require. You might find that they actually request less.

Finally regarding your question as to when to approach them, I would do this pretty much straight away. It is not necessary to wait for her physical residency card to be granted, although you might want to take with you documents to prove that she will soon obtain her card.

I hope this information is helpful.