На весілля приїдуть і онука, і онук нашого дідуся.

Because на весілля expresses motion to an event, so весілля is in the accusative case.

A very common Ukrainian pattern is:

• на + accusative = movement to an event/place
• на + locative = being at that event/place

So:

• на весілля = to the wedding
• на весіллі = at the wedding

One thing that can confuse learners is that весілля looks the same in the nominative and accusative singular, so you do not see a form change here.

Приїдуть is the 3rd person plural future form of приїхати.

So grammatically it means they will arrive / they will come.

A useful nuance:

• приїхати usually suggests arriving by transport or from some distance
• прийти usually suggests coming on foot

So приїдуть is not just a random future form of come; it specifically comes from a verb of arrival that often implies travel.

Because приїхати is perfective, its future form is a simple future:

• приїду, приїдеш, приїде, приїдемо, приїдете, приїдуть

This is a standard Ukrainian pattern: і ..., і ...

It means both ... and ...

So:

• і онука, і онук = both the granddaughter and the grandson

A single і often just means and, but the repeated structure gives the sense of adding both items emphatically.

You will also see similar patterns with other words, for example:

• і мама, і тато
• і вчитель, і учень

Because together they form a compound subject.

Even though each noun is singular by itself:

• онука = granddaughter
• онук = grandson

the whole subject means two people, so Ukrainian uses a plural verb:

• приїдуть = will arrive / will come for they

This works just like English:

• The granddaughter and the grandson are coming not
• The granddaughter and the grandson is coming

Because this part means of our grandfather, so Ukrainian uses the genitive case.

The nouns онука and онук are being identified by relationship:

• granddaughter of our grandfather
• grandson of our grandfather

So дідусь changes to the genitive singular:

• nominative: дідусь
• genitive: дідуся

And the possessive adjective changes too:

• nominative: наш
• genitive masculine animate: нашого

So:

• нашого дідуся = of our grandfather

It describes both.

So the structure is understood as:

• і онука нашого дідуся, і онук нашого дідуся

Ukrainian often avoids repeating the same phrase when it clearly applies to both items in a pair.

So the sentence is not saying:

• the granddaughter, and the grandson of our grandfather

It is understood as:

• the granddaughter of our grandfather and the grandson of our grandfather

It is both a case form and part of a slightly different noun.

Ukrainian has:

• дід = grandfather / old man
• дідусь = grandfather, often warmer or more family-like

In this sentence the base form is дідусь, and дідуся is its genitive singular form.

So:

• nominative: дідусь
• genitive: дідуся

This form sounds natural and affectionate, not unusually childish.

Yes, taken by itself, онука can be ambiguous.

It can be:

• онука = granddaughter in the nominative singular
• онука = grandson in the genitive singular, from онук

But in this sentence it is clearly granddaughter, because:

1. it is paired with і онук
2. both nouns are part of the subject
3. the structure і ..., і ... strongly suggests two coordinated nominative nouns

So here:

• онука = granddaughter
• онук = grandson

Yes. Ukrainian word order is fairly flexible, and changing it often changes emphasis, not the core meaning.

For example, you could also say:

• І онука, і онук нашого дідуся приїдуть на весілля.

That version puts more focus on who will come.

The original sentence:

• На весілля приїдуть і онука, і онук нашого дідуся.

starts with На весілля, which makes the destination/topic come first. This is very natural in Ukrainian.

So the word order here is not random; it helps organize the information.