I've been giving the cut direct and I have no ideal why.

[pierrotlunaire0]

Or, here is another take: the person is a bully, and they are enjoying the power of making you uncomfortable and stirring up the family gatherings.

Adopt a pleasant smile, and give then a nod of recognition.  Let their ignoring you roll off of you.  Even if their motive is something more truly deserving of sympathy than my bullying motive, then your cool smile will make it easier for them in the future to resume friendly relations.

[Bijou]

What do members think of this?  Is it too direct?  With DH permission and participation (since it is his family) ask them privately if they are upset with you about something and if so, can it be resolved?

[zyrs]

I'm with pierrotlunaire0.  Give them a pleasant smile and a nod of recognition.  Then turn your back or walk away.

[Otterpop]

I'm with zyrs and pierrelunaire0 as well.  Be cool and polite.  They'll look like, and ARE, the offending ones.

I also agree with your mom that they're upset you're "stealing the spotlight."  I've seen this happen more than once and it is irrational, but very real to them.  Your offence could be being more prosperous, having more children, being smarter, prettier, etc.  Some people are jealous, twisted and manipulative and will act as though you've done something wrong.  Do not take the "guilty person" roll as another poster said (evil me would super secretly enjoy being envied). 

[TootsNYC]

One thing that might help you navigate this in the future is to keep your focus on the OTHER people in the room. And to act in a way that minimizes the drama for them.

If you can set aside your own hurt feelings and hurt pride and the sting of being disrespected, then you can focus on "how to make this exchange be low drama for the other people in the room?"

So when you're in a grouping that Chilly Relative joins, keep a mild smile, wait for them to finish, and then resume your conversation. Much the way you'd act if--oh, you and a friend went to a restaurant for lunch and the manager of the restaurant came to talk to your friend about the dinner party she was planning there next month.
   You'd withdraw a little bit--enough to not be "sticking your nose in" to their conversation; you'd keep a pleasant but noncommital expression on your face; you'd wait patiently for them to finish. If it seemed they were going to take a long time, you'd say, "excuse me" to your friend and step away for a while.
    Or if you were at a convention w/ a friend and their boss came up to talk business for a bit--you might wait to see if they would finish quickly, or else you'd go off somewhere else.

That's your strategy. And it might be easier to adopt it if you embrace the "separation" between you and Chilly Relative, and then focus on how to keep things low-drama for everyone else.

Oh, and if anyone brings it up, say "Chilly Relative seems to prefer not to speak with us, so we oblige him/her."