I'm so sorry for your loss, what a huge and sudden responsibility on the heels of such a shocking loss.
Your baby brother is angry and lashing out. He needs a lot more hugs and snuggling now. When he gets angry and says rude things it's time to take him to a quiet place and talk. I know it feels more appropriate to give him a time out, or swat, but that's going to make it all much worse. He's been abandoned by his parents and he's all messed up over it. It's going to take a truckload of love to help him recover from such a massive loss at such a delicate age.
I know your plate is full, but you need to rally your sister and brother to share the love and help each other through this. Even though they may be coping better than your baby brother, they no doubt need the extra hugs and "I Love You's" and "I'm Proud Of You's" too, everyone does.
If there are any grief recovery groups for kids in your area you might want to bring your siblings. A social services office would have a list of such groups/services. Better to offer support now than deal with troubled teens in a few more years.
Best of luck to you and your growing family.