I would ask the buyers if they would rather work with their aunt and brother-in-law. 

If your brother were a gynecologist, would you use him?  Some people don't use relatives for professional services - others do.  Never assume, always ask.

Are you going to be working with them? If yes, seeya. If no, fine let's roll.

Build the relationship, ask the questions,get answers.  

More detailed information is needed. You need to "dig in" and find out what that information means to your, perhaps - perhaps not - future client relationship with these people. Why are they even talking to you, anyway?

I'd be finding out what their situation is... not everyone wants to work with a relative. I'd ask them why they wanted to speak with me if they had a close relative in the business. I recently listed and sold a home for sellers who had first hired their stepmother and stepbrother. They fired them ultimately and hired me. I

Neither. They are working with YOU, that says it all. 

I work with many people that have relatives in the business. They prefer me for privacy reasons and because they know I'm more experienced. They also know we'll have a professional relationship and experience.

Miss Wanda Kubat-Nerdin - Wanda Can! - I need more information. 

The short answer is ... I will dig in and ask questions.  Where are they licensed - in another state or are they local? 

If local, will they be using their relatives to make their purchase? 

Many of my clients have relatives in the business and a few were licensed agents way back when.  Asking questions and digging in will most likely get you valuable information.  Sometimes it gives clues if they are hedging and not being fully transparent. 

Do you already have a buyer's brokerage agreement?  If so no worries, if not, time to get one or stop working with them.

"That is good to hear. Why are you not using them?"
"they are still on probation because they violated some duty and got braclets."
"If you follow my directions there will be no braclets in our future. Now, let's review this agreement and get your 'John Hancock'."

I don't miss a beat and don't go by anyone else's script.  My first question is "are the local?"  If they are then a probe to see how good they are and how committed they are to working with them.  If they are out of area I explain that I would never want a listing in an area that I don't know the area and local rules and regulation. When push comes to shove I'll resort to offering their relative a referral fee.

"Tell me more... are they here in this area?  Do they know that you are looking for a house?  Are you planning to work with them?"

Time to clarify why they called you to help them and to make sure who they really want to help them. Listen and then decide what you chose.

Further conversation (my ear not mouth) is required

I'd ask where? Maybe they're licensed in another state? Need more info.

I would ask them about it just to get it off your mind. That way you can either carry on or leave them at the next light.

Lynnea Miller keeps it simple!

What Lynnea Miller said.  Find out if you need to fish or cut bait.

I have found that the best way to the truth of anything, is to put it out there. I'd probably say something like, "Oh, that's great! Were you planning to use them?"

If yes, or if no, I know what the next steps are.

"And why did you contact me?".  Their answer and any conversation will likely determine what happens next.  

A piece of a pie is better than no pie. Depends on the value of your time.

I hope they have signed the agreement to work with you.

If they have come to me for help, I would not read too much into it if they sign a Buyer agency agreement engaging my services.

I had a seller ask if I would give her son a referral, he was a licensed agent. I said of course no problem.

Well Mrs Buyer, you made a wise choice choosing our team over them...

If you have a properly written Buyer Agency agreement it means that they CHOSE to work with you. Clarification if probably in order to avoid a last-minute disappointment.

I would say, "Oh, that's cool, where do they practice?" Maybe they're just trying to get a referral fee or maybe they don't want to work with the aunt and B-I-L. We have agency agreements here and I would make sure they're under agency before doing any work for them. If they do want to work with the family members, I would let them know they'll need to go to them for all questions, etc.