The "Sambandham" did not really involve nair families in general. The Namboodiris had a custom that only the eldest son could marry a kanyaka from a Nambudiri family. The rest of the male issues were prohibited from marrying or keeping any kind of contact with even their own sisters. The custom used to be that the younger male issues were called "Apfan Namboodiris" (अप्फन्) and after 'samaavartanam' an apfan Nambudiri had no access to the female quarters of their own native "illam" and except for their mother and grandmother, these apfan Nambudiris had to talk to even their blood sisters, athais, athai pATTies, etc. by turning their face away from them and standing a certain distance away.
Because of the erudition, intellect, the higher social status as also the materially rich status of most Nambudiri illams, the royal families of the many petty principalities of this region, who followed the Marumakkatthaayam system of inheritance, considered it a status symbol to have "Sambandham" of learned and rich Nambudiris with the womenfolk in their royal houses. Slowly this custom was adopted by the aristocratic nair families (FYI, even under the label 'Nair' there were hundreds of sub-divisions stretching down to 'viLakkitthala nAyar' (barber)) and the Sambandham privilege was not for any nair.
Most of the so-called royal households in old Kerala, including the Travancore rajas are considered as "Sudras" and not as Kshatriyas.