How to help an arguing couple?

my brother and my sister in law often argue. sometimes about money issues, sometimes about being able to do something on a school night (my brother is going to college) or sometimes about buying cigarrettes or whatever. do you have any ideas on how i could help them out? i know they kinda got to sort stuff out on their own, but it just feels weird when im with them and they start arguing about things.

like one time, i was in my brothers truck and my sister in law turned the radio on, and my brother didnt like the song, so he wanted to change it to his cd, but she said to leave it, and they kept switching it back and forth, untill i finally stepped in and seemed to act like the mature one (even though im only 16 and they’re both in their 20s) and said “if you cant decide on what to listen to, then you cant listen to anything” and i just shut the radio off..

any ideas on how i could help them? maybe a marriage counseling book or something? i dont really know what to do.. or do i just need to leave them alone to sort their own things out?