kazoku ga hueru mae ni, hituyouna mono wo sukosizutu sonaeru tumori desu.

Because 増える is an intransitive verb. It means to increase / to grow, and the thing that increases is the subject, so it takes が.

• 家族が増える = the family increases / the family grows
• Not 家族を増える because 増える does not take a direct object

If you wanted a transitive version, you would use 増やす:

• 家族を増やす = to increase the family / to add to the family

These are a common intransitive/transitive pair:

• 増える = to increase, to grow
  Something increases on its own, or is described as increasing.
• 増やす = to increase something
  Someone actively causes the increase.

Examples:

• 人口が増える = The population increases
• 人口を増やす = to increase the population

In your sentence, 家族が増える means before the family grows, not before I increase the family.

When a verb comes before 前, it describes the action/event that happens before something else.

So:

• 増える前に = before [the family] increases
• literally: before increasing

A plain non-past verb before 前に often means before that action happens.

Compare:

• 寝る前に = before sleeping
• 行く前に = before going
• 家族が増える前に = before the family grows

The に marks a point in time.

So:

• 前 = before
• 前に = before (at the time before...)

In many time expressions, に helps show when something happens.

Here, 家族が増える前に sets the time frame for the main action:

• Before the family grows, ...

Because 必要 is a na-adjective, not an i-adjective.

When a na-adjective directly modifies a noun, you use な:

• 必要な物 = necessary things
• 静かな部屋 = a quiet room
• 便利なアプリ = a convenient app

So 必要な is the correct form before 物.

Yes. 物 here means things / items / belongings / stuff, depending on context.

So 必要な物 means:

• necessary things
• the things we need
• necessary items

In this sentence, it probably refers to practical things that should be prepared in advance.

Also, in everyday writing, you may often see もの instead of 物. The meaning is the same here; the difference is mainly one of writing style.

少しずつ means little by little, bit by bit, or gradually.

• 少し = a little
• ずつ adds the sense of in small amounts at a time or one by one / little by little

So 必要な物を少しずつ備える means the speaker plans to prepare the needed things gradually, not all at once.

Examples:

• 毎日少しずつ勉強する = study a little every day
• 少しずつ集める = collect little by little

備える means something like:

• to prepare
• to provide
• to equip oneself with
• to make provisions for

It often has a nuance of preparing in advance for a future need or situation.

So in this sentence, 備える sounds like getting the necessary things ready ahead of time, which fits very well.

By contrast:

• 準備する is a more general to prepare
• 備える can sound a little more like laying in what is needed, making sure you are equipped, or preparing for what is coming

So 備える is very natural here.

～つもりです means intend to do or plan to do.

It attaches to the dictionary form of a verb for a positive intention:

• 備えるつもりです = I intend to prepare
• 行くつもりです = I intend to go

So:

• 必要な物を少しずつ備えるつもりです = I intend to gradually prepare the necessary things

This expresses the speaker’s current intention or plan.

Because つもりです attaches to the plain form of the verb.

For affirmative intention:

• dictionary form + つもりです

Examples:

• 買うつもりです = I intend to buy
• 勉強するつもりです = I intend to study
• 備えるつもりです = I intend to prepare

For negative intention:

• ない form + つもりです
• 行かないつもりです = I do not intend to go

Japanese often omits subjects when they are clear from context.

Here, つもりです usually refers to the speaker’s intention unless another subject is clearly stated. So the implied subject is most naturally:

• I
• or sometimes we, depending on context

So the sentence naturally means something like:

• Before the family grows, I plan to prepare the necessary things little by little.

Japanese does this very often, especially with first-person statements.

Literally, yes: the family increases / the family grows.

In real usage, though, it often implies:

• a baby is coming
• someone is joining the household
• the family will have one more member

So it is a natural and somewhat indirect way to talk about an addition to the family. The exact reason is not stated, but in many contexts it strongly suggests childbirth or pregnancy.

It has this basic structure:

• 家族が増える前に = before the family grows
• 必要な物を = the necessary things
• 少しずつ = little by little
• 備える = prepare
• つもりです = intend to

So the sentence is:

• [Before the family grows], [I intend] [to prepare] [the necessary things] [little by little].

Japanese often puts the time-setting phrase first, and the main statement comes at the end.

Because つもりです is the polite form.

Compare:

• 備えるつもりだ = plain / casual
• 備えるつもりです = polite

So the sentence is written in a polite style, which is very common in textbooks and general conversation.

Usually 家族が増える前に is the more natural choice here.

That is because が marks 家族 as the subject of 増える. The sentence is describing the event the family grows.

Using は would change the nuance. It might sound more contrastive or topical, as if you were specifically talking about the family, perhaps in contrast with something else.

So for a straightforward statement of what happens before the event, が is the normal choice.