Det här gäller inte bara mig, utan också min syster.

Det här means this.

Swedish uses:

• den här with a common-gender noun: den här boken = this book
• det här with a neuter noun or when this stands on its own

In this sentence, det här stands by itself and refers to this matter / this situation / this statement, so det här is the natural choice.

So:

• Det här gäller inte bara mig = This doesn’t apply only to me
• But if you had a noun, you might say: Den här regeln gäller inte bara mig = This rule doesn’t apply only to me

Gäller is the present tense of gälla.

Here, gälla means:

• to apply to
• to concern
• to be valid for

So Det här gäller inte bara mig means something like:

• This doesn’t just apply to me
• This doesn’t concern only me

It is a very common verb in Swedish for rules, situations, statements, and general relevance.

Because this is a normal Swedish main clause, and Swedish follows the V2 rule: the finite verb usually comes in the second position.

Here the structure is:

• Det här = subject
• gäller = finite verb
• inte = negation
• bara mig = only me

So the sentence is built like:

• Det här
  • gäller
    • inte bara mig

This is normal Swedish word order.

Compare:

• Det här gäller inte bara mig = This applies not only to me
• Inte bara jag gäller det här would be wrong for this meaning

Because mig is the object form of jag.

Swedish personal pronouns work like English:

• jag = I
• mig = me

Since gälla here takes the person as an object-like complement, Swedish uses mig, not jag.

So:

• Det gäller mig = It concerns me
• not Det gäller jag

The same pattern appears with other pronouns:

• dig = you
• honom = him
• henne = her
• oss = us
• dem = them

This is a very common Swedish pattern meaning not only ... but also ...

In your sentence:

• inte bara mig = not only me
• utan också min syster = but also my sister

So the full pattern is:

inte bara X, utan också Y

Examples:

• Det gäller inte bara mig, utan också min syster.
• Hon talar inte bara svenska, utan också engelska.
• Vi tänker inte bara på priset, utan också på kvaliteten.

A useful point: the two parts usually match each other in structure. Here both mig and min syster are noun phrases, which makes the sentence balanced and natural.

Because utan också is the standard pairing in this construction:

• inte bara ..., utan också ...

So Swedish normally says:

• inte bara X, utan också Y

This is the most straightforward equivalent of not only X, but also Y.

You may also see utan även in slightly more formal style:

• Det här gäller inte bara mig, utan även min syster.

Both are correct, but utan också is very common and natural.

Because in Swedish, a possessive like min, din, hans, hennes, vår is normally followed by the noun in its indefinite form.

So:

• min syster = my sister
• not min systern

Compare:

• syster = sister
• systern = the sister
• min syster = my sister

This is a very important Swedish pattern:

• min bok = my book
• mitt hus = my house
• mina vänner = my friends

Yes, you could.

Detta also means this, but it is usually:

• more formal
• more written
• less conversational

So:

• Det här gäller inte bara mig = natural, everyday Swedish
• Detta gäller inte bara mig = correct, but more formal or written

A native speaker in normal conversation would usually prefer det här.

Not always.

In modern Swedish, commas are often used less than in English. A comma before utan can appear, especially to make the sentence clearer or to mark a pause, but many people would also write this sentence without a comma:

• Det här gäller inte bara mig utan också min syster.

So:

• with comma = perfectly fine
• without comma = also perfectly fine and very common

Yes.

You can say:

• Det här gäller inte bara mig, utan också min syster.
• Det här gäller inte bara mig, utan även min syster.

Both are correct.

A rough difference:

• också = very common, everyday also
• även = a bit more formal or written in some contexts

In this sentence, both sound natural, but också may feel a little more conversational.

It is most commonly used for:

• rules
• statements
• situations
• conditions
• general points

So it often means this applies to... or this concerns...

Examples:

• Den här regeln gäller alla. = This rule applies to everyone.
• Det här gäller mig också. = This applies to me too.
• Det gäller särskilt barn. = This concerns children especially.

So in your sentence, det här refers to some general matter, not a physical object. That is why gäller fits very naturally.

Yes. That is a very good literal translation.

Depending on context, English could also say:

• This doesn’t apply only to me, but also to my sister
• This concerns not only me, but also my sister
• This is not just about me, but also about my sister

The Swedish sentence itself is flexible enough to match several English phrasings, depending on what det här refers to in context.