Welcome No Peace: I'm so sorry that your son deals with suicidal thoughts. Sounds like he might be inconsistent in taking him meds. Do you think it is because he just forgets, or could it be issues with side effects? How often does he see his psychiatrist? Does he meet with a psychologist between times?
When someone in your life deals with suicidal thoughts, you have to take it seriously, even if they have seemed to bluff in the past.
The information at the link below can be helpful, when someone in your life deals with suicidal thoughts. Has your son ever called/texted a suicide hotline? It would be good if he had some emergency numbers in his phone. That way, it would be easier for him to use in an emergency. The one link below is for texting for crisis support.
SUICIDE AND CRISIS SUPPORT https://bpdfamily.com/pdfs/safety_first.pdfThe website below is a selfhelp website for DBT Therapy. It is a type of therapy commonly used for people with BPD. The page that the link takes you to is a helpful list of things to do for distress tolerance. It should be helpful for your sister, if she is open to learning some skills.
PANIC LIST FOR DISTRESS TOLERANCE www.dbtselfhelp.com/html/panic_list.htmlTEXT CRISIS LINEwww.crisistextline.org/how-it-works/