I had brilliant comments all line up, until I got to the post that said that you yourself are Jewish, Pruf.
First off, you’ll never dissuade someone from Orthodoxy by quoting text at them. They will outquote you every time, and there are far more texts supporting their point of view than yours.
Second, please note that there are major distinctions between so-called “modern Orthodox” and ultra-right wing Hassidim. Basically, Orthodox Jews accept the validity of the Bible and all the rabbinic commentary. The body of laws (including commentary) is called halakha, and addresses almost every aspect of life – how to eat, how to wear clothes, etc. Orthodox Jews accept the unbroken line of tradition as binding. Conservative Jews believe that the line of tradition can be modified; Reform Jews don’t find the traditions binding.
For many Conservative or Reform Jewish families, sad to say, the idea of a son or daughter marrying into an Orthodox faith is repulsive. The opposite is also true, that many view marriage with Conservative or Reform as tantamount to conversion. The Nazis didn’t make such distinctions, and the mingled ashes of Orthodox, Reform, Conservative, secular, and other branches should be a constant reminder… but sadly are not.
OK, there are some Hassidic groups that are very cult-like and sect-like, hovering around a charismatic rebbe/guru, and cutting off relationship with the family. There are some groups that reject the existence of the State of Israel, for instance. Such groups are few and small in number; if your sister is thinking of joining one of them, you might want to consider some drastic intervention.
But if you’re talking mainstream Orthodox (whether traditional or modern), my opinion is that you haven’t got the right to interfere. To the contrary, you are better off letting her explore whatever avenues her heart leads her down… and let her come to her own conclusions. Your antagonism will only push her further down that path, for the wrong reasons. Support her in investigating, let her know your personal stance, but let her know that you respect her ability to make up her own mind.
I can guarantee that if you try to tell her that Orthodox is “silly” or “nonsense,” she’ll want to prove that you are wrong, and you’ll push her further in the direction you don’t want her to go.
Frankly, the things that you quote – traditional observance of the sabbath, keeping kosher, not wearing revealing clothing – don’t sound very objectionable to me.