[RPG] How to convert a god from 3.5 to 5e

Pre-warning - I have little experience as a DM dealing with difficult players, but I'm getting pretty used to dealing with difficult tables in general.

A couple of things we found really handy for our horde of uncooperative players:

Talk it out / Remind him

You said in the comments that:

I have talked to him about it before. He seems understanding until a situation occurs where it basically sets it off

So he may just need a gentle, at table reminder about the discussion you've had. A simple prod in the right direction might be all he needs to cool his head. DnD can get pretty intense at times, and it could just simply slip his mind as he gets into it.
Players really getting into your campaign is what you want as a DM, as long as they're getting into in good spirit.

Is the table okay with this behavior?

Is it just you that's bothered by this? How your table reacts to his behavior could also dictate how you handle it.

During our romps with difficult players, one player refused to cooperate with the DM to the point we almost had to kick him out. But the DM pulled me aside, and asked me to respond to every that player said as in character.

If your brother is heavily into role playing, responding in-kind and keeping it to your characters may help stem the OOC poor behavior.

I.E - If he's complaining about the Gods not lending a hand, a NPC (or other player) could take that at face value. "[Gods Name Here] wouldn't care about a single [town/city/country/]! Our lifes are nothing compared to the grand scheme of things. The immortal plane is beyond our understanding..."

If all else fails, are you guys that right group for him?

You also said that:

He generally seems much kinder to other dms, doesn't have the remarks or throws tantrums to them. (He also is my older brother)

So it could very much be as simple as -Younger brother has to do it my way rawr- or that the tables play-style and campaign just doesn't fit him.

Saying that you play to his level, but then having a level 15 player challenge a lv 20 NPC, says to me that maybe he doesn't want that much of a challenge.

He could be looking for something a bit more cruisy, or maybe he just really really wanted to play pirate captain.

But either way, if you can't find a middle ground with him, maybe have another look to see if he matches your table. It could be as simple as a few small tweaks to the campaign to keep everyone happy.

Use visual reminders

Sometimes, a player/DM like this only needs a visual cue that you've discussed in advance to remind them that they're engaging in behaviour you've previously talked about. Since this person is open and receptive, but keeps falling into habitual metagaming, start using a visual cue to remind them.

I recommend a pack of playing cards. Just tell him/her that the cards are meant to serve as a reminder to try to pull back the metagaming. Every time they start, just hand them a card. If the person is as receptive as you said, they'll check themselves immediately.

After time, this will no longer be necessary. You'll have effectively trained them out of a bad habit. Be sure to reward said behaviour with some cold beer to reinforce the change. :)

This advice comes from a lot of experience dealing with players who used to DM and actually falling into this same habit myself. Visual cues can be very subtle, and very powerful.