Game Of Familia: Kazoku Senki

The protagonist along with his step-mother,

and 2 step-sisters(step-aunts) are summoned into another world

where demi-humans are the rulers as saviors,

but its not to fight against a demon lord,

its to fight an Undead Apocalypse monsters called

“Dead Mell” that have overrun the world

and that can only be killed by using weapons made with holy silver.
For that, a familia is made (group of 4 people)

with them being the swordmaster that uses the holy sword,

the guardian that uses the power of earth to protect,

the healer that use the power of water to heal

and the black mage that uses the power of fire to destroy.
The role of the guardian, the mage and the

healer is to support the swordmaster that can only kill the monsters.

But to use magic powers one must offer part

of your being as a sacrifice to form a contract with spirits,

and the higher the sacrifice, the higher the power.

Now they must choose who are the 3 that

need to sacrifice part of their bodies to gain power

and who will wield the holy sword.