Okay so this deck was a lot harder to put together than I expected. I've been wanting to put together a Rin and Seri, Inseparable deck for awhile. While I don't feel like there are a lot of good Cat and Dog options for Rin and Seri, I feel like I put together the best with what I had to work with.

Note: If you are building this on Arena, Leonin Warleader is still a legal card on Arena. So swap out Maned Serval for Warleader.

This deck is half aggro, half midrange. It revolves around playing a lot of cats and dogs for Rin and Seri to create cat and dog tokens and then either swarming them out or OTKing them with Rin and Seri's ability. I won't go over each card, but I will make note of certain cards below.

The Cats:

Bronzehide Lion can make itself Indestructible, but if it does die, it can come back as an Enchantment on one of your creatures to give it its ability. Cubwarden makes more cats when it mutates. There are a couple of other mutate creatures in the deck. So you can make a few cats from it. Feline Sovereign is one of the cat lords along with giving the cats the ability to destroy artifacts and enchantments. The Prot Dogs isn't really applicable. Keeper of Fables is handy because the only human in this deck is Alpine Houndmaster. So it gives the deck some efficiency. Pridemalkin gives everything with a +1/+1 counter Trample. Not too terrible. There are a couple of ways to put counters on creatures here. Prowling Felidar is a nice Vigilance creature that gives itself a +1/+1 counter through Landfall. Kaheera is the other cat lord and also gives all our cats Vigilance.

The Dogs:

Savior sacs itself to give another creature Indestructible. Bolt Hound is a nice Haste creature with an okay on-attack trigger. Alpine Watchdog is a generic 2/2, but has Vigilance. Igneous Cur can buff itself. Rage-Hound can bring itself back from the graveyard with +1/+1 counters. Pack Leader is the dog lord that negates combat damage to our other dogs when it attacks. Magmutt has a ping effect on it.

The Support:

Felidar Retreat lets us make more cats or give all creatures +1/+1 counters on Landfall. Syncs with Pridemalkin. Not much to say about Henge that people don't already know. Ramp, Lifegain, Counters, Efficiency, and cheaper to cast based on total power. One of the best Eldraine Legendary Artifacts. Lukka gives a little gas if we start to run out. His -2 is great for digging stuff out of our library. Vivien lets us cast creatures from the top of our deck, makes chump blockers, and does a psuedo-Cascade effect for her -2. Houndmaster lets us thin our deck of Apline Watchdog and Igneous Cur.

Conclusion:

Overall, this is a pretty decent deck that can be difficult to stop once it starts going. I just hope there will be some more cats and (especially) dogs in upcoming sets *cough*Kaldheim*cough* that will make Rin and Seri better. As it stands, Rin and Seri are probably better in EDH than Brawl.