I would say it solely depends on your relationship with your SIL. My SIL and I are best friends. We tell each other EVERYTHING. I also am not afraid to tell her like it is. I honestly think that if anything happened between DH and I we'd still be just as close (she's closer to me than anyone in her family including DH and my MIL/FIL).
With that said, because of our relationship I WOULD be sure to intervene. But that is because of the relationship we have. As for other family members, I did recently intervene with my dad. He has made POOR financial choices over the years including not paying taxes etc. He's 64 years old and will work until the day he dies. I only told my siblings (I'm the baby of 7 - oldest is 15 years older than me) after he showed me his paycheck as a JOKE. Not a joke when your gross is $9,000 a month and your check is less than $1,000 after paying back taxes, regular taxes and paying back a 401k loan. My siblings and I intervened and I learned A LOT when we did. He doesn't want help and he will continue to live foolishly. There were many tears and arguing but it ultimately solved NOTHING.
I guess what I'm saying is, it doesn't hurt to have a conversation about it if it's something YOU'RE comfortable with. If you're not then don't bother. Sometimes family members need to hear it from the people that care most about them - sometimes it works, sometimes it doesn't. My mom is CONSTANTLY telling me/nagging me to be careful how we spend our money (my parents are not married btw) and has done it for YEARS. It took me a while to actually listen and I appreciate her nagging now.
There definitely is a fine line between what's appropriate and what's not and I believe only you can determine the relationship you have with your SIL to decide to say anything or not.