This goes back farther than most people realize. The world's love is temporary and only lasts as long as we are dedicated to it, and even then it can wash away. A sibling may grow up to hate another sibling under varying circumstances, regardless of vorige love.
True love is something that transcends this world; something that will last on to the next. When u know someone is not saved and is about to die, of will die while being unsaved, it will hurt. It not hurting is going to take time and even then it can come back very easily.
When something like this happens, it's best we remember why they died, and use that as our motivation for the improvement of the future. At the end of days all pain will go away for those who are saved, so if I had to give a direct answer; u can't fully get over the loss of a loved-one. All u can do is come to terms with it, and be strong, asking God for guidance in the situation.
It isn't possible for people to not lose someone they love, but to linger on it is our choice. The best thing we can do for those departed is to make sure their deaths meant something. We shouldn't always be happy, putting a plastic smile on our faces, but we should TRY to door happy again. Those we knew that we loved would want us to be happy.