Ta przecena kończy się dziś wieczorem, dlatego moja siostra już jedzie do sklepu.

Because przecena is a feminine singular noun, and the demonstrative this must agree with it in gender, number, and case.

• ta = feminine singular
• ten = masculine singular
• to = neuter singular

So:

• ta przecena = this sale / this discount

Since przecena is the subject of the sentence, it is in the nominative case, and ta matches that.

Przecena usually means a price reduction, discount, or sale.

In this sentence, Ta przecena kończy się dziś wieczorem means something like:

• This sale ends this evening
• This discount period ends this evening

A useful distinction:

• przecena = reduction in price / markdown
• wyprzedaż = sale, often a larger store sale or clearance sale

In many contexts, both can be translated as sale in English.

Because kończyć się means to come to an end / to end.

Compare:

• kończyć coś = to finish something
• kończyć się = to end / to be ending

So:

• Kończę pracę. = I am finishing work.
• Praca kończy się o piątej. = Work ends at five.

In your sentence:

• Ta przecena kończy się... = This sale ends...

The się is very important here; without it, the meaning changes.

In Polish, the present tense is often used for scheduled or expected future events, just as in English.

Compare English:

• The sale ends tonight.

That is grammatically present, but it refers to the future.

Polish does the same:

• Ta przecena kończy się dziś wieczorem.

So even though it happens later, the present tense is natural because it describes something fixed or imminent.

Dziś wieczorem means this evening / tonight.

Here:

• dziś = today
• wieczorem = in the evening / this evening

The form wieczorem is the instrumental case of wieczór. In Polish, some time expressions use the instrumental without a preposition:

• rano = in the morning
• wieczorem = in the evening
• nocą = at night

So dziś wieczorem is a fixed and very common expression.

Yes. Dziś and dzisiaj both mean today.

• dziś is a bit shorter and very common
• dzisiaj is also common and fully natural

So these are both possible:

• Ta przecena kończy się dziś wieczorem.
• Ta przecena kończy się dzisiaj wieczorem.

The meaning does not really change.

Dlatego means therefore, that’s why, or for that reason.

It connects the first clause with the result in the second clause:

• The sale ends this evening, therefore my sister is already going to the store.

The comma is used because two clauses are being linked:

• Ta przecena kończy się dziś wieczorem, dlatego moja siostra już jedzie do sklepu.

This is very normal punctuation in Polish.

Because moja siostra is the subject of the second clause.

The subject is in the nominative case:

• moja siostra = my sister

If it were a direct object, you would use a different case:

• Widzę moją siostrę. = I see my sister.

But here your sister is the one doing the action:

• moja siostra już jedzie... = my sister is already going...

So nominative is required.

This is a very common Polish motion-verb question.

• iść / idzie = to go on foot
• jechać / jedzie = to go by vehicle or to travel/ride

So:

• Moja siostra idzie do sklepu. = My sister is going to the store on foot.
• Moja siostra jedzie do sklepu. = My sister is going to the store by car/bus/tram/etc.

The sentence uses jedzie, so it suggests she is traveling there not on foot.

Już usually means already.

So:

• moja siostra jedzie do sklepu = my sister is going to the store
• moja siostra już jedzie do sklepu = my sister is already on her way to the store

It adds the idea that the action has started sooner than expected, or that it is happening now because of the sale ending soon.

In this sentence, już helps show urgency:

• the sale ends tonight,
• so she is already going.

Do takes the genitive case, so sklep changes to sklepu.

• sklep = store
• do sklepu = to the store

This is the normal pattern when expressing movement to a place:

• do domu = to home
• do szkoły = to school
• do sklepu = to the store

So the structure is:

• jechać do + genitive

That is why you get do sklepu, not do sklep.

Yes, Polish word order is more flexible than English, because case endings help show what each word is doing.

The given sentence is neutral and natural:

• Ta przecena kończy się dziś wieczorem, dlatego moja siostra już jedzie do sklepu.

But you could change the order slightly for emphasis, for example:

• Ta przecena kończy się dziś wieczorem, dlatego już moja siostra jedzie do sklepu.
• Ta przecena kończy się dziś wieczorem, dlatego do sklepu już jedzie moja siostra.

These alternatives are possible, but they sound more marked or emphatic. For a learner, the original version is the safest and most natural.

Yes, that is an excellent translation.

A very natural full translation would be:

• This sale ends this evening, so my sister is already going to the store.
• This discount ends tonight, so my sister is already heading to the store.
• This sale ends tonight; that’s why my sister is already on her way to the store.

All of these capture the Polish well.