Pater dicit sibi nihil referre utrum semita an limes longior sit, dummodo ratis tuta maneat.

sibi is the reflexive dative pronoun, and it refers back to the subject of the main verb, pater.

So:

• Pater dicit = Father says
• sibi nihil referre = that it matters nothing to him

Latin uses sibi rather than ei when the pronoun refers back to the subject of the governing clause. If it referred to someone else, Latin would use a non-reflexive form such as ei or illi.

Here nihil referre is an idiomatic expression meaning:

• to matter nothing
• to make no difference

So sibi nihil referre means it makes no difference to him.

This is one of those places where you should not translate referre by its more basic dictionary sense such as bring back or report. In this idiom, refert / referre means it matters.

Compare:

• quid refert? = what does it matter?
• nihil refert = it does not matter at all

Because it depends on dicit.

After verbs of saying, thinking, knowing, perceiving, and so on, Latin often uses indirect statement. In indirect statement, a finite verb in the original thought becomes an infinitive.

So the direct idea would be something like:

• sibi nihil refert = it matters nothing to him

But after pater dicit, Latin changes that to:

• pater dicit sibi nihil referre = father says that it matters nothing to him

This is standard reported-speech syntax in Latin.

Because refert is being used impersonally.

In many indirect statements, you get an accusative subject + infinitive, as in:

• dicit puerum venire = he says that the boy is coming

But nihil refert is not a normal personal statement like the boy is coming. It is an impersonal expression: it matters / it makes a difference.

So Latin does not need a personal accusative subject here. Instead, the thing that may or may not matter is expressed by the clause:

• utrum semita an limes longior sit

In other words, whether the path or the boundary-road is longer is the content that doesn’t matter.

It is an indirect question.

The pair utrum ... an ... means:

• whether ... or ...

So:

• utrum semita an limes longior sit = whether the path or the track/boundary-road is longer

This indirect question depends on referre:

• sibi nihil referre utrum ...
  = that it makes no difference to him whether ...

Because indirect questions in Latin take the subjunctive.

That is the main reason. The clause is not using the subjunctive because the speaker is being especially doubtful or hypothetical; it is simply the normal grammar of an indirect question.

So:

• direct question: utrum semita an limes longior est?
• indirect question: utrum semita an limes longior sit

Also, because the main verb is present (dicit), the present subjunctive sit fits the usual sequence.

They are both nominative singular.

They are the two alternative subjects of sit in the indirect question:

• semita ... longior sit = the path is longer
• limes ... longior sit = the track/boundary-road is longer

The clause asks which one is longer, so both nouns appear in the nominative as possible subjects.

longior is nominative singular comparative, and it can agree with either noun because each noun is singular.

It may look surprising because:

• semita is feminine
• limes is masculine

But in the comparative, the nominative singular masculine and feminine are the same form:

• longior = masculine singular or feminine singular

So longior works perfectly with either noun:

• semita longior
• limes longior

If the comparison were plural, you would see different forms.

dummodo means provided that, so long as, or as long as only.

It introduces a proviso or condition:

• dummodo ratis tuta maneat = provided that the raft remains safe

So father is saying that the question of which route is longer does not matter, provided that the raft stays safe.

This word often has the sense of that is the one thing that matters.

Because dummodo regularly introduces a clause with the subjunctive.

So:

• dummodo ... maneat = provided that ... remains

This is normal Latin usage. The subjunctive here does not necessarily mean strong unreality; it is simply the expected mood after dummodo in this kind of proviso clause.

ratis is a noun meaning raft.

Here it is nominative singular, the subject of maneat:

• ratis ... maneat = the raft remains

And tuta agrees with it:

• ratis tuta = the safe raft or more naturally the raft safe

Since ratis is feminine, the adjective is feminine too:

• tuta, not tutus

A good way is to build it in layers:

1. Pater dicit
   = Father says

2. sibi nihil referre
   = that it makes no difference to him

3. utrum semita an limes longior sit
   = whether the path or the track is longer

4. dummodo ratis tuta maneat
   = provided that the raft remains safe

So the structure is:

• main clause: Pater dicit
• indirect statement: sibi nihil referre
• indirect question depending on referre: utrum ... an ... sit
• proviso clause: dummodo ... maneat

That layering is very typical of Latin: once you identify each clause type, the sentence becomes much easier to follow.