I am so deeply sorry to hear of this news, it must be incredibly difficult for your sister and for you.
Some herbs that could be of emotional support in grieving a lost child are: motherwort, hawthorn, rose, linden, lemon balm & mimosa (Albizia julibrissin). There are many others as well that might call out to you or your sister more, I’d stay start exploring with these and some other nervines, to see which ones resonate the most. Are there any herbs/plants that you have on hand or have worked with while they are still growing outside?
More ideas could be Reishi mushroom, chamomile, lavender, & lady’s mantle etc.
I send great warmth and care during your grieving. May the plants hold space and care for you both.