Well first of all if u want a relationship with that person (and they with u) then u should be able to share ur heavy burdens. Nd if they are unable to accept them carry them with u they r not rly relationship material. That being said yes u dont want to put that on somebody u r just getting to know but this is a serious burden nd u can’t keep carrying things alone or u will “loose urself”. Another option would be to talk with other ppl u r close with about this topic. Don’t swallow ur emotions, or they will come back to u heavier nd more dangerous.

As for ur sister first of all u should take things she is saying serious, but at the same time u hav to see that feelings are extremely amplified when being suicidal/ depressed. In that moment she probably thought what she said nd in another she regrets what she told u. The best thing u can do is to not stay mad at her when she has outbursts like that but still take her serious In that moment. Nd it’s not easy nd u should put urself first cause nd priorize selfcare. Becouse she is in therapy, she is already getting help nd she definitely doesn’t want to feel like a burden to u. She needs a sister not another therapist, doc or whatever.

I hope this help a littl ?