Twin weapon made of sisterstone...

[Laclale♪]

If same grade object of dusk sisterstone and scarlet sisterstone are within 15 feet, they'll make reciprocal field.

And twin weapon, due to hit within same turn, needs to be in same distance(For most of them).

Then... oops what I suppose to say

[Cordell Kintner]

This is a Blood Lords specific item, and should be in that forum.

Also, Twin weapons are intended to be used by the same person, and the reaction only works if an ally is hit.

[Taja the Barbarian]

This Item may contain spoilers from the Blood Lords Adventure Path:

Relevant text blocks:

Sisterstone (Item 0+) wrote:

This Item may contain spoilers from the Blood Lords Adventure Path

Rare, Precious
Source Pathfinder #183: Field of Maidens pg. 79

Sisterstone is a term used for two closely related ores infused by the spiritual runoff in the Field of Maidens, dusk sisterstone and scarlet sisterstone. They have the same physical properties except for color—dusk sisterstone is a pale orange while scarlet sisterstone is orange-red. When near an object made of the other type of sisterstone, they both begin exuding spiritual energy that repels undead.

When an object made of dusk sisterstone is within 15 feet of an object made of scarlet sisterstone of the same grade, they create a reciprocal field. Any undead creature adjacent to one of the objects when the field is formed, or who moves adjacent to one while the field is active, becomes enfeebled 1 unless it succeeds at a DC 15 Fortitude save. This lasts until the undead creature is no longer adjacent to either of the items or until the field's removed (typically by the objects ceasing to be within 15 feet of one another). The DC increases to 22 if both items are standard grade or better, and to 33 if they're both high grade. Unworked chunks and ingots don't create a reciprocal field. A single object can't be made of both types of sisterstone.

Sisterstone Weapon (Item 3+) wrote:

This Item may contain spoilers from the Blood Lords Adventure Path

Rare
Source Pathfinder #183: Field of Maidens pg. 80
Usage varies by weapon; Bulk — varies by weapon

A weapon with a sisterstone striking surface can be charged with undead-destroying energy in response to violence from an undead creature. A creature wielding a sisterstone weapon gains the following activation. Only melee weapons can be made of sisterstone.

Activate command; Trigger The ally is critically hit by a Strike from an undead creature you can see; Requirements A reciprocal field is active between this weapon and an ally's sisterstone armor or sisterstone weapon; Effect Your weapon leaps out with positive energy in retaliation. Strike the triggering undead with the weapon, and your Strike deals 2 additional positive damage. The positive damage increases to 4 if your sisterstone weapon is standard grade, or 6 if it's high grade. If the triggering undead is beyond the reach of your sisterstone weapon, your Strike doesn't deal its base damage, but can still deal the positive damage.

And since the term 'ally' is used:

Chapter 9: Playing the Game / General Rules / Effects / Targets wrote:

Source Core Rulebook pg. 455 3.0

Some effects require you to choose specific targets. Targeting can be difficult or impossible if your chosen creature is undetected by you, if the creature doesn’t match restrictions on who you can target, or if some other ability prevents it from being targeted.

Some effects require a target to be willing. Only you can decide whether your PC is willing, and the GM decides whether an NPC is willing. Even if you or your character don’t know what the effect is, such as if your character is unconscious, you still decide if you’re willing.

Some effects target or require an ally, or otherwise refer to an ally. This must be someone on your side, often another PC, but it might be a bystander you are trying to protect. You are not your own ally. If it isn’t clear, the GM decides who counts as an ally or an enemy.

Offhand, I'm guessing the intended question might have been "If I wield one weapon of each type, can I use the reaction when I get crit?" to which the answer is No because you do not count as your own ally.

[Laclale♪]

Aha, then two set of twin! If each twin are equipped by ally, both can use reciprocal field from each other!

But if used in this way, what reciprocal field's DC will be used?

[Cordell Kintner]

Being a twin weapon doesn't matter at all. If you have a dusk weapon, and an ally with a scarlet weapon/armor is crit, you can use the reaction. Same goes for an ally, if they have a scarlet weapon and you a dusk one, they can use the reaction. It doesn't matter who has which version, you can both use the reaction as long as you're within 15ft of each other.

Also, Twin weapons aren't any cheaper to make than two standard weapons. Two Sisterstone Sawtooth Sabers cost the same as two Sisterstone Daggers.

[Taja the Barbarian]

Cordell Kintner wrote:

Being a twin weapon doesn't matter at all. If you have a dusk weapon, and an ally with a scarlet weapon/armor is crit, you can use the reaction. Same goes for an ally, if they have a scarlet weapon and you a dusk one, they can use the reaction. It doesn't matter who has which version, you can both use the reaction as long as you're within 15ft of each other.

Also, Twin weapons aren't any cheaper to make than two standard weapons. Two Sisterstone Sawtooth Sabers cost the same as two Sisterstone Daggers.

English isn't the OP's primary language so I'm guessing 'twin' might just mean 'pair' as in 'one dusk sisterstone weapon and one scarlet sisterstone weapon to create a reciprocal field' rather than the actual Twin trait.

As for which DC to use for the reciprocal field (to avoid being Enfeebled 1 while adjacent to one of the weapons), I'm guessing you'd just use the strongest: Given the DCs, I doubt the low grade field would actually penalize any of your foes if you actually have access to multiple high grade items...