Si hace buen tiempo, mis tíos vendrán al campo con nosotros.

Because si without an accent means if.

By contrast:

• sí = yes
• sí can also be used in expressions meaning himself / herself / itself, as in para sí

So in this sentence, Si hace buen tiempo... means If the weather is good...

This is a very common Spanish weather expression. Spanish often uses hacer with general weather conditions:

• hace calor = it’s hot
• hace frío = it’s cold
• hace sol = it’s sunny
• hace mal tiempo = the weather is bad
• hace buen tiempo = the weather is good

So hace here does not literally mean makes in the way an English speaker might expect. It is just the normal verb used in this kind of expression.

Because after si in a real, possible condition, Spanish normally uses the present indicative, not the future.

So the standard pattern is:

• Si + present tense, future tense

For example:

• Si tengo tiempo, iré.
• Si viene, lo veremos.
• Si hace buen tiempo, mis tíos vendrán...

Using si hará buen tiempo would sound wrong in standard Spanish for this kind of sentence.

Because bueno shortens to buen before a singular masculine noun.

Since tiempo is masculine singular, you say:

• buen tiempo

Compare:

• un buen libro
• un buen día
• el tiempo es bueno ← here it stays bueno because it is not directly before the noun

So:

• before the noun: buen tiempo
• after the noun or with ser: el tiempo es bueno

Vendrán is the future tense of venir.

It means they will come.

The full future conjugation of venir is:

• vendré
• vendrás
• vendrá
• vendremos
• vendréis
• vendrán

So in this sentence, mis tíos vendrán means my uncles will come.

Because venir is irregular in the future tense.

Instead of using the regular stem, it uses vendr-:

• venir → vendré, vendrás, vendrá, vendremos, vendréis, vendrán

This is something you simply have to learn as an irregular pattern. A few other common verbs do something similar in the future:

• tener → tendré
• salir → saldré
• poner → pondré
• poder → podré
• haber → habré

The accent mark shows where the stress goes: ven-drán.

In the future tense, the ellos/ellas/ustedes form ends in -án, and that syllable is stressed:

• vendrán
• llegarán
• comerán

So the accent helps both with spelling and pronunciation.

Because mis is the plural form of mi.

Spanish possessives like mi / mis agree with the thing possessed in number, not with the owner.

• mi tío = my uncle
• mis tíos = my uncles

So because tíos is plural, you need mis.

Because al is the contraction of a + el.

• a el campo → al campo

This contraction is required in standard Spanish.

So:

• ir al campo
• venir al campo

But note that this only happens with a + el. For example:

• a la playa stays a la playa
• a los pueblos stays a los pueblos

In this sentence, al campo usually means to the countryside or out to the country.

In Spain, el campo often refers to the rural area, not necessarily a single field. Depending on context, it could suggest:

• the countryside in general
• a rural family property
• a country house or country area where the speaker is going

So it is broader than just a field.

Because after a preposition like con, Spanish uses the prepositional pronoun form, not the clitic form nos.

So you get:

• conmigo
• contigo
• con él / con ella
• con nosotros / con nosotras
• con ellos / con ellas

That is why con nosotros is correct.

Nosotros is used for:

• a group of males
• a mixed group of males and females
• a group whose gender is not being specified

Nosotras is used only if the whole group is female.

So if the speaker is talking about a mixed group, or just using the default form, con nosotros is correct.

Yes, it is normal and correct here.

When a si clause comes first, Spanish usually separates it from the main clause with a comma:

• Si hace buen tiempo, mis tíos vendrán al campo con nosotros.

If the order is reversed, the comma is often omitted:

• Mis tíos vendrán al campo con nosotros si hace buen tiempo.

So the comma in your sentence is standard punctuation.

Yes, Spanish word order is fairly flexible, although the original sentence sounds very natural and neutral.

For example, these are also possible depending on emphasis:

• Si hace buen tiempo, vendrán mis tíos al campo con nosotros.
• Mis tíos vendrán al campo con nosotros si hace buen tiempo.

The version you have is a very good standard way to say it. It puts the condition first and then gives the main result.