Not spam, but a challenge: ツンツンしてた小悪魔妹が嫁になるまでデレた理由

Breaking that down:

(((tsuntsun shite-ta) koakuma-imouto) ga
((yome ni naru) made) dereta) wake

(Note: the last word is the kanji for riyuu, but as you can see on Amazon, it’s glossed wake; both mean “reason/explanation”, but at the moment I can’t really explain what difference in nuance they’re playing with here)

Stripped to essentials, the base sentence is imouto ga dereta, “younger sister was ‘lovestruck’” . This complete sentence modifies wake (as what’s often called an attributive verb; your textbook may vary), making it “the reason younger sister was lovestruck”.

Now for the rest: koakuma “little devil” modifies imouto, or, more precisely, I think it should be read as a compound noun koakumamai, “little-devil younger sister”. Tsuntsun shite ita, “(someone) had been ‘aloof’” is another attributive verb, modifying our complex little sister. That leaves us with the particle made, “until”, which provides the condition that ended her tsuntsun nature: yome ni naru, “(she) becomes a bride”.

The base sentence is past tense, so the awkwardly-precise result is: “The reason my used-to-be-aloof-until-she-married little-devil younger sister became lovestruck.”

In more natural English, perhaps “How marriage made my bitchy little sister sweet”.

[Update: I read a bit of the back cover blurb, and the protagonist says his ideal bride is a not-related-by-blood little sister, so given the genre and the cover art, perhaps that should be “How marrying me made my bitchy little pricktease of a step-sister put out”.]