I noticed that crisis is also fem., so can I take it (as a very general rule of thumb) that sis words are generally fem.?
That seems as good as any of the other rules of thumb that we use to guess genders. =) "Análisis" is masculine (as are derivatives like "psicoanálisis"), "énfasis" is masculine, and "oasis" is masculine, but so far all the other -sis words I can think of are feminine.
-sis = a suffix appearing in loanwords from Greek, where it was used to form from verbs, abstract nouns of action, process, state, condition, etc. eg The crisis is from Greek η κρίση also feminine gender.
Doesn't end in an A so why is it LA? Makes no sense since there are female and male words or whateva
The "feminine words end with 'a'" thing is only good as a very rough rule of thumb, for times when you don't know and you have to guess. It's not a hard-and-fast rule. There are lots of feminine words that don't end in "a" (including "la tesis"), and lots of words that do end in "a" but aren't feminine ("el sistema," "el problema," etc.).
The best thing to do is, when you learn a new word, memorize it with the article. That way, you know it's "la tesis" or "el problema" or "la mano," and you'll know what the gender is.