Could this be Akame Ga Kill!?, which was adapted to a 24 episode anime in 2014?
Tatsumi is a fighter who, accompanied by his two childhood friends, sets off to the Capital in search of a way to make money to assist his poverty-stricken village. After being separated from his friends in a bandit attack, Tatsumi unsuccessfully attempts to enlist in the army and is swindled out of his money in the Capital. He is taken in by a noble family, but when an assassin group called Night Raid attacks, he learns that his noble hosts actually intended to torture and kill him as they had done with his friends.
As a result, he joins Night Raid, which consists of the swordswoman Akame, the beastly fighter Leone, the sniper girl Mine, the scissor-wielding Sheele, the string manipulator Lubbock, the armored warrior Bulat, and their leader Najenda, a former general of the imperial army. Night Raid is also part of the revolutionary forces assembled to overthrow the prime minister Honest who is manipulating the young emperor for his personal gain despite the rest of the nation falling to poverty and strife.
The members of Night Raid carry Teigu (帝具, anime: Imperial Arms), unique weaponry created 900 years ago out of extremely rare materials as well as legendary animals called Danger Beasts (危険種, kikenshu). The power of the Teigu is so overwhelming that it is said that when two Teigu users fight each other, one of them is bound to die.
Tatsumi does wield a sword, and gains blue armor when he brings on his power. Several characters wield guns in Night Raid and among their opposition, and the Teigu are essentially demon-powered. I cannot find reference to a sister transforming into a blob monster, although Kurome is an enemy who is the sister to deuteragonist Akame, the character the series is named after.
Trailer
Found with a search for anime sword transform guns blood "fight * sister"