As an older sibling

You gotta save these bitches when they in imminent danger, and then find the lawn chair to watch them flounder in bad relationships, while casually tossing a pool noodle at them when they surface. Cause if they don't take it, you can't force them to, much as it fucking sucks.

Also, from experience, trying to force your opinion without proper context of the situation, both makes you an ass, and makes the sibling or other person not trust you to open up to. Which then means that they will throw the pool noodle back at you, while purposefully inhaling water, out of spite and not wanting to be judged for their bad judgment.

Point is, be a good and supportive older sibling, and know when you need to ask hard questions, and when it is best to shut ya mouth. Also, in situations with domestic abuse, it is far more complicated, I have helped many a sibling, parent, and others with these kinda things. Also lot of it is empathizing with their struggles, and asking questions that encourage them to think and come to conclusions about their relationships on their own.