Marriagetoxin

From Viz:
As a deadly assassin from the poison clan, Gero swears he’ll never settle down. That is, until the family orders his sister to produce the poison clan’s next heir. Refusing to let his sister abandon her chance at true love with her girlfriend, Gero desperately turns to marriage swindler Mei Kinosaki for relationship advice. Can Kinosaki turn this trained killer into a charming husband-to-be?

The prequel oneshot Hyper Hard Special Mission is included in Vol 5.