If YOU are the one paying their living expenses and they make a costly mistake which YOU will have to pay to bail them out of. By all means let them know.
If THEY pay their bills and they make a costly mistake. If they ask for your input, by all means give them your honest opinion. If they don’t ask, you shouldn’t tell.
If you disagree and think that they are mistaken with where they daven, send their kids to school, go on vacation, etc. These issues are NOT costly mistakes for you to have an opinion to offer.