First, go to your Extended John Marston Animations folder (animation fixes), then open human_character_upper.ymt with a notepad and follow the steps

Arthur:
Step 1: Use ctrl + F to find <!--ARTHUR_MORGAN-->

Step 2: Change <UNK_MEMBER_0x6978C5D6/> to <UNK_MEMBER_0x6978C5D6>default</UNK_MEMBER_0x6978C5D6>

John:
Step 1: Use ctrl + F to find <!--JOHN_MARSTON-->

Step 2: Change <UNK_MEMBER_0x6978C5D6/> to <UNK_MEMBER_0x6978C5D6>default</UNK_MEMBER_0x6978C5D6>

Step 3: Find <UNK_MEMBER_0x18140ED7> and paste this block of code BELOW it:
 <Item>
          <UNK_MEMBER_0x7EFFBD94>0x4399538B</UNK_MEMBER_0x7EFFBD94>
          <UNK_MEMBER_0xE532732B>0xE67E2EAD</UNK_MEMBER_0xE532732B>
        </Item>

After you're done, delete human_character_upper.ymt from this mod.