Uk Family Visa Or Plain Tourist Visa?

Hi Guys n Gals,

My question is when applying for my Thai wife's visa can I apply for a family visit visa or do they need to be blood relatives?

We are married in Thailand and have the documents translated into English. Also Guarenteur/sponsor etc etc.

On the form I would fill out relationship - husbands mother (i.e. mother in law).

Is this good enough or would I be better off applying for a tourist visa for my wife and a family visa for my Thai (no English passport) son so he can write on his application : relationship - grandmother.

Thanks,

Keelan