This LOL. I think "ツンツンしてた小悪魔妹が嫁になるまでデレた理由"/"tsun-tsun shiteta koakuma imouto ga yome ni naru made dere ta ryuu" translates approximately as "My Small Devil of a Little Sister would Even be ok to marry if It makes her Nicer" (need J.'s help here).

J.P. also twittered that "koakuma" is a term for a fashion style. I somehow missed the -kei part.

UPDATE: J helps out with a suggestion: don't try to outsmart yourself by looking at まで as "even", simply use it as "until". Still, it's odd. J's full solution is "How marrying me made my bitchy little pricktease of a step-sister put out".