There's lots of generational trauma in my family, so my parents aren't the most emotionally healthy people in the world (albeit in very different ways from one another) and I heard quite a few zingers over the years, but the ones that stuck with me the most regarding mental health were said by my dad, such as when I was 11 and he told me how much of a burden I was to everybody because of my anxiety and said "Shame on you!" over and over again while I was having an anxiety attack, and that time he told me to just kill myself already – "Don't try, just fucking do it already!" He also got really irritated with me for telling my stepmom that I wanted to kill myself when I was 13, because apparently it was "bad timing". Sorry that my suicidal thoughts aren't more convenient?
Not sure if in-laws count, but an honourable mention would be when my MIL said to me regarding my C-PTSD, "What traumatized you isn't happening to you right now, so you should just live in the moment and be happy!" Not how C-PTSD works at all, but okay.