(Spoilers for the Lucas segment)
When the player defeats the Mutant Marguerite boss fight, they get a phone call from Lucas, one of the main antagonists of the game. He tells the player to "look in the fridge" in which there is a severed and heavily mutilated head saying "the pig is waiting for you in the dissection room, bitch".
In said dissection room, the player goes through an unskippable first-person-cutscene of their character reaching into the rest of the body to pull out a key.
There are a few depictions of mannequins used for unknown purposes scattered around the houses. They are simply background details, however, and do not hold any significance to gameplay or story.
The player can inject themself with steroids (which increase max health). This is completely optional. There are non-optional sections where the player must inject others with an antidote. These scenes can be found in the end of the final battle with Jack, after escaping Lucas' area, and towards the end of the game where the player will inject the antidote into the main antagonist after finding them.
Yes, the father Jack is mentioned to get drunk a lot as said by the mother, Marguerite. There are also two bottles of beer present at the dinner table and Jack does drink some at one point.
the circumstances of and before their death are not shown, but the remains of several animals appear in the game, often just left to rot or otherwise placed in grotesque ways; they don’t appear to have been killed for food.
They're included in the bugs used by the mother. They only appear in her section of the game. There's a scene or two where spiders will inevitable jump on you, but they can be generally avoided when free roaming. You'll find them covering lockers and they will jump on you if you hit them while close. Using guns, the flamethrower (which I'd recommend for this), and bombs from a distance will prevent them from jumping on you while still damaging them.
The player can use a knife to damage enemies. One of the first enemies will use a knife to attack the player. The player's arm and foot is cut off (also early game). The player has a word cut into their skin (found in the "Happy Birthday" tape).
Yes, there is a scene where Jack drowns Marguerite in a bathtub, repeatedly pushing her under the water until the end of the sequence where her body goes limp, seemingly dying.
A character (first person) is burned alive (I'll include a summary of the events leading up to it below in case you'd like to just skip over it while playing). Jack, the father, is inside a car when it crashed and explodes into flames, but he just walks out (very much on fire), seemingly unharmed. The player can also use a flamethrower to damage enemies, but none of them will actually burn to death (except for the mold monsters, but I don't count them as being "alive").
(SMALL SPOILERS THAT DON'T AFFECT THE GAME THAT MUCH)
While exploring Lucas' room, the player will find a tape labeled "Happy Birthday." After inserting this tape into one of the TVs scattered around the house, they will play as someone else caught in Lucas' trap. After solving all of the puzzles, the player will place a candle on a birthday cake. The cake will erupt in flames, making both the player and the entire room catch fire. Without a way to escape or put themselves out, the player will die an agonizing death. It includes lots of screaming.
A man's head gets cut in half (horizontally, while still on body). Someone gets shot in the head and they reappear with a large portion of their head missing and regenerating. When attacking the common monsters, their heads might explode if you get a headshot.
One of the villains pulls a few of their fingernails off. I'll leave a summary of the events leading up to it in case you wish to skip it in-game.
(SMALL SPOILER, NOT TOO IMPORTANT TO THE PLOT)
While in the son's (Lucas') section of the game, the player will encounter a TV. The TV will turn on and Lucas will talk to Ethan. Lucas will say something around the lines of "now THIS is amazing" before pulling a few of his fingernails off with a pair of pliers. He's interrupted by someone and stops soon after.
Both parents die. (SPOILERS AHEAD, I'LL TRY TO BE VAGUE)
Jack Baker "dies" several times over, but he does actually perish eventually. Margarite Baker only dies once. Neither of them are good parents, as they are literally monsters. Near the end of the game, Jack (lucid and not crazy) pretty much consents to being killed.
There are showers in the game, but none of them are used. The closest thing to a shower are the shower faucets found in Lucas' puzzle room, and there's no "scene" to that.
Technically not, but the Baker family has the same behavior as one who is possessed (not very clearly until later in the game). Mia is the only one that really looks like she's possessed. Early in the game, Mia will clearly switch to a violent version of her self.
The player can choose to inject themselves with steroids (to increase their max health), but that's completely optional. The player will have to use syringes a few other times though in order to progress the story. I'll list those times here: the end of the last fight with Jack, after beating Jack and escaping Lucas' area, and near the end of the game after the player locates Eveline in the original area from the beginning of the game.
There are several small, closed spaces in the game that the player has to go through. Some are holes underground, some are narrow paths between walls, etc.
No relationships, but the son, Lucas, is said in one of the notes to be staring at his sister Zoe while she's doing yoga. This is the only mention though, and there is nothing any further than this.
Yes, the mother Marguerite is drowned in a bathtub by her husband Jack. She dies at the end of the scene, her body going limp, but she does return in another scene. This happens in the daughters DLC.