Hi Kristina!
It is a pretty neat feature, and it’s easy to use once you get the hang of it! 
You can make separate replies by using the “quotes” features- which is done by highlighting the text you want to quote, then clicking the “quote” button.
For a full guide (with helpful screenshots!) you can visit here:
→ An Easy Guide for Multiple Replies 
Another great question! Magickal ethics vary by tradition- most Wiccans choose to follow the Wiccan Rede and the Threefold Law, but not all witches are Wiccans- other traditions, such as Druidry or Voodoo, will have their own doctrines and guides to ethics that should be followed (if you wish to be part of that religion or tradition).
Eclectic Witches (such as myself) and all those who follow multiple paths or blaze a trail entirely their own are free to set their own magickal ethics in their practice. When you make your own magickal practice, you are the only one who can decide what is “right” or “wrong” for you. Then it’s just a matter of practicing in a way that honors and respects your own beliefs.
For more about magickal ethics, @MeganB has a great video on the topic that she shared in Ethical Witchcraft 
What Are Your Thoughts?
I am so sorry to hear about what happened to your cousin- my heart goes out to you and your family, Kristina 
About your spell- when it comes to magick, it is all about intention- wishing bad things on someone would be a different spell than calling on karma or justice to be the judge. Others may disagree (again, the topic of magickal ethics can vary quite a bit from one witch to another!), but I personally would not consider calling on justice or the hand of karma to be Baneful/Black Magick. In my eyes, it’s the same as calling for help or notifying the authorities when you see a crime- you’re just bringing it to the attention of higher powers who will hopefully come to help and bring justice for all.
Just my two cents offered up for your consideration! I would encourage you to research the traditions that call to you and hear their stances about magickal ethics. The more you learn, the better prepared you will be to find what works best for you and your beliefs 
Wishing you all the best- blessed be! 