So very sorry for you all Suzie - so terrible xxx
I know it's not the same, but when my darling Dad died, we found his diaries after, not deep personal things, just a record of their everyday life. These gave me so much comfort for the first few months. It was like bringing him nearer and recalling happy times.
I just wonder if some way of raising money together for a charity in her name would help you all at all? Or maybe the little ones could make a keepsake book with all their happy memories of their cousin, they could draw pictures, add photos, and write down their particular memories of their cousin. If the book was there, they might feel sad at times and just want to add to it. Your sister might in the future, take a little comfort from it too.
I also think the idea of a garden for her would be lovely. The girls could plant forget me nots, and there could be a wind chime.
For you and your sister and family, I am sure you never get over the loss of a child, but these ideas may help the children a little. Take care xx