I think in their life stories there are some similarities, yet some differences. I'm not an expert on Sissi, having only read one book about her ( there seem not be many good books about her in English), but my take is the following.The marrying at a young age, tragic death, and unhappy marriage are true. Both were also famous beauties were restless and never seemed happy, in her later years Sissi moved from place to place, and some how, that might have been Princess Diana's fate too, had she lived longer. She seemed to be sort of drifting after her divorce, but one difference with Sissi, is she had more purpose, so she might not have become the way Sissi was, never finding a home, just traveling from one place to another. Sissi was very vain, and I don't think Princess Diana was.
Princess Diana did have an eating disorder, and some people said Sissi did too, but a completely different one than that of Princess Diana. Sissi was said to starve herself to remain young and thin. One difference they had was as mothers. Sissi wasn't maternal, at all, whereas Princess Diana was, and identified herself as such. Both their husband didn't understand them, and they both died younger ( what was Sissi's age at death?), in the last years of a century, Sissi was murdered/ asssinated, and Princess Diana wasn't although some have said that she was. The worlds they inhabited were different. But, Sissi in later years barely lived in Austria I believe, and I think she died abroad, as did Princess Diana, another thing in common. Sissi was way more self interested than Princess Diana, but then Sissi had more of an aimless life, perhaps. I don't know much else, but that is my opinion.