Smarty pants heroine: Ahn Eun-jin.
Clueless Alien Goofy husband : Taec-yeon.
Helpful passive-aggresive MIL : Kim Mi-kyung.
The ex? Comic relief please. That'll be Yang Kyung-won.
Kooky Uncle Auntie: Park Ji-young.

Smarty pants heroine: Park Eun-bin
Clueless alien: Kim Soo-hyun, obviously
Clingy comic relief ex: Choi Jin-hyuk in Shin Hye-sun's body
Mom: Kim Mi-kyung
Kooky uncle: Kim Won-hae

Bridesmaids:
#1 - the claustrophobic Chae Soo-bin
#2 - the "was dragged into the wedding party" Kim So-hyun
#3 and 4 - the perpetually hangry Seo Hye-won and her forever cheerful twin sister, Joo Hyun-young.
#5 Rowoon.

I'm lazy so I'll just swap in the entire cast of Because This Life Is My First (switch Kim Byeong-ok from dad to uncle). For the ex, maybe Lee Joon?

So My Love From the Star + The Martian = Korean Weekender in Space?

Whoever came up with this one gets a bow.

I don't know about any of the casting for this drama, but if you are going to have a reference to The Martian, you need a cameo of Matt Damon and Sarah Silverman (maybe at the wedding) singing "their song" together.

Not sure about the other characters but I would love to see Son Seok Ku's take as the alien groom.

Kim Mi-kyung is everybody's mum, except I see Kim Mi-kyung as perfect for someone who cleans so that things break, but not obviously passive-aggressive. She can do it, - as we saw in Knight Flower - but mostly she is just aggressive and would beat up her SIL if he didn't put food enough on the table.
Could it be Kim Hae-sook, maybe? I could see her cleaning like a stampeding herd of bisons and being passive-aggressive as caustic soda with artificial sweetener.

Lee Kwang-Soo as the alien groom
Joo Woo-Jae groom’s father
Jang Doyeon groom’s mother
Kim Ji-Yu as groom’s grandfather
Park So-Yi as groom’s grandmother
Lee Se-Hee as the bride
Ra Mi Ran as the bride’s mother
Kim Hee Won as the bride’s uncle
Kang Ha Neul as co-worker ex boyfriend of bride

The aliens are humanoid species whose main traits are: they have octopus skin heads with tiny tentacle beards; they have tiny suction cups on their fingers; they grow up to their mature age then their bodies slowly revert to their child state; every six hours they must wear an aquarium helmet so their brains do not turn to dust.

The bride is a part of a research team for the colonization of Mars. Original colony was powered by small nuclear generators fueling a cluster of connected hexagonal buildings. It was decided that Mars had to be terra formed by exploding many polar nuclear bombs to build a Martian atmosphere. Her ex-boyfriend is for the idea and the bride was violently opposed to it. The groom was a space envoy to help Earth solve its planetary pollution issues but spent most of his time seducing his bride with his unusual, sensual body parts.

The groom’s parents troll the bride’s uncle with thoughts about making Earth “their home world” and the uncle takes that as these “aquarium fish” lusting after the planet’s great oceans to breed and conquer the world (even though salt water is toxic to them). The groom’s grandparents are the only people who have any common sense and try to get along with everyone.

The bride’s mother is a nosy clean freak. When she cleans the grooms’ helmet with solvent, the next time he uses it he has a violent allergic reaction turning his skin into a orangey cheetah pattern. The uncle begins to call him “Orange Pus” which aggravates the groom to no end, but he holds his tongue for the sake of his bride. It becomes worse when they take potatoes to grow outside the hydroponic lab into the native soil (it turns the Russets into orange potatoes.) As they consume these Martian potatoes, the humans begin to change their appearance into orangish spuds - - - which revolts the groom and his family to the point of leaving the mutants on Mars after their ship recharges.