Life can be cruel. How we react to the occasion molds us into the people we are. While nothing can replace your sister, remember to treasure her memory and all the little things she meant to you. As for as your thoughtless relatives, try to have some compassion. Everyone reacts differently to unexpected death and while they may have been inappropriate, they could be hurting inside and trying to hide it or just wanted to distract themselves.
Good luck on your return to work. Hopefully, you will be able to concentrate and the routine of work will be somewhat reassuring for you.
My thoughts and prayers remain with you in this time.