Hon’ble Supreme Court has held that, a concubine is not a relative to the husband of the knife, with regards to IPC 498A.

The word ‘cruelty” having been defined in terms of the aforesaid explanation, no other meaning can be attributed thereto. Living with another woman may be an act of cruelty on the part of the husband for the purpose of judicial separation or dissolution of marriage but the same, in our opinion, would not attract the wrath of Section 498A of the Indian Penal Code.

Meaning or definition of “relative”:

In the absence of any statutory definition, the term ‘relative’ must be
assigned a meaning as is commonly understood. Ordinarily it would include
father, mother, husband or wife, son, daughter, brother, sister, nephew or
niece, grandson or grand-daughter of an individual or the spouse of any
person. The meaning of the word ‘relative’ would depend upon the nature
of the statute. It principally includes a person related by blood, marriage or
adoption.

The word ‘relative’ has been defined in P. Ramanatha Aiyar
Advanced Law Lexicon – Volume 4, 3rd Edition as under :-
“Relative, “RELATIVE” includes any person related by blood, marriage or adoption. [Lunacy Act ].
The expression “REALTIVE” means a husband wife, ancestor, lineal descendant, brother or sister.
[Estate Duty Act].

“RELATIVE” means in relation to the deceased,
a) the wife or husband of the deceased;
b) the father, mother, children, uncles and
aunts of the deceased, and
c) any issue of any person falling, within either
of the preceding sub-clauses and the other
party to a marriage with any such person or
issue [Estate Duty Act].
A person shall be deemed to be a relative of another if, and only if, –
a) they are the members of a Hindu undivided family, or
b) they are husband and wife; or
c) the one is related to the other in the manner indicated in Schedule I-A [Companies Act, 1956].

“RELATIVE” in relation to an individual means –
a) The mother, father, husband or wife of the individual, or
b) a son, daughter, brother, sister, nephew or niece of the individual, or
c) a grandson or grand-daughter of the individual, or
d) the spouse of any person referred to in subclause (b) [Income tax Act].

“RELATIVE” means –
1) spouse of the person ;
2) brother or sister of the person ;
3) brother or sister of the spouse of the person;
4) any lineal ascendant or descendant of the person;
5) any lineal ascendant or descendant of the spouse of the person;
[Narcotic Drugs and Psychotropic Substances Act].”

From Para 18,

By no stretch of imagination a girl friend or even a concubine in an etymological sense would be a ‘relative’. The word ‘relative’ brings within its purview a status. Such a status must be conferred either by blood or marriage or adoption. If no marriage has taken place, the question of one being relative of another would not arise.