Oh Tambo - what awful news - I'm sorry to hear this. My sympathies to both yourself and to your family.
Just go with your thoughts and let them flow onto the page, especially if you were close. but if you are struggling, some ideas would be:
My family and I are truly saddened by the loss of (Name). Our thoughts and hearts are with you at this time and if you need us for anything at all, we are here for you, even if it is just to talk. I can't imagine what you are feeling right now, so please reach out to me in your time of need - I am only a phone call away.
Then if you feel up to it, you could put a little about your memories of your cousins daughter and how she made you smile - and if you are religious, perhaps offer a prayer.
Very difficult to put your feelings into prose at this time, but just do a draft first, read and if it feels ok, write it on the card.
Once again - my condolences Tambo xx