Worried about my sister help?

So heres the deal, my identical twin sister has recently gained about a stone or so in weight, i dont know how to tell her i think she needs to loose weight, all she seems to think about is chocolate rather than all the fun things we used to do like going shopping together, in fact she hates shopping now, she finds things like this tiresome and stressful. since she has gotten fatter she is really grouchy and breathless compared to how she used to be. how do i tell her these things