Lately some relatives and friends started to ask me who he is
and what his relationship with my wife is. I started me sitting
back and thinking...
Before your post, I was thinking what was
the catalysis and your last paragraph says it clearly - folks who
questioned status quo. In some ways, the intent of your question
was almost to seek if such behaviours were a norm. Nope - it is
uncommon - given the intensity and depth of your description.
However, uncommon does not imply impossibility and certainly does
not suggest infidelity.
At the end of the day, whatever arrangement made is an outcome
of choices chosen in view of our needs. For example, if she pubs
with him on a Saturday night and you choose to stay at home, then
if this arrangement works for you in a practical sense -
economically speaking, this exchange is efficient. However,
emotionally speaking, if it is uneasy for you, then being
'efficient' is not something that you need.
Since his presence existed way before the birth of your
relationship, then there must be something good about you (and the
relationship) and something 'missing' between them that has never
allowed development of a relationship to manifest. An analogy would
be: if I have kerosene on the floor, it does not necessarily means
a fire will start; without lighting the match, it is merely
kerosene on the floor.
A good start would be to get
involved with your wife's life or find something common to do
together. Somehow, I get the sense that this element is rather
subtle in your marriage. You don't have to see it as a mechanism to
control/keep an eye on her, but rather, learn to have quality time
beyond that of child rearing. Having fun
in love marriage is also crucial, especially when the couple is
becoming overly enmeshed with the concept of raising children at
the expense of their interaction pattern.
Not raising any alarm at the
moment - but it's good to do a review of your relationship once in
a while so as to embrace growth and find greater fulfillment in
it. 
Cheers
