Frater et soror tandem consentiunt se matri parere debere, quia discordia omnibus nocet.

Because frater et soror together make a compound subject: brother and sister. In Latin, as in English, a compound subject normally takes a plural verb.

So:

• frater = brother
• soror = sister
• frater et soror = brother and sister
• consentiunt = they agree

Tandem means at last, finally, or in the end.

It adds the idea that this agreement did not happen immediately. The brother and sister have presumably been in conflict, and only finally do they agree.

This is an indirect statement construction, which is very common in Latin.

After a verb like consentiunt (they agree), Latin often uses:

• an accusative subject
• plus an infinitive

instead of an English that-clause.

So:

• se = the subject of the indirect statement, in the accusative
• debere = to ought, to be obliged
• parere = to obey

Put together:

• se matri parere debere = that they ought to obey their mother

A more literal unpacking is:

• se = themselves / they
• matri = to their mother
• parere = to obey
• debere = to ought

Se is the reflexive pronoun, and here it refers back to the main subject, frater et soror.

So se means themselves here.

A useful point: se can be either singular or plural in meaning, depending on context. The form stays the same.

Here the subject is plural, so:

• se = themselves

Because this is an indirect statement.

In an indirect statement, Latin does not use a normal finite verb like debent. Instead, it uses an infinitive:

• debere = to ought / to be obliged

So the main finite verb of the sentence is:

• consentiunt = they agree

And what they agree is expressed by the infinitive clause:

• se matri parere debere = that they ought to obey their mother

If Latin used debent, it would no longer be the normal indirect statement construction.

Because parere depends on debere.

Latin often uses debere + infinitive to express obligation:

• debere parere = to ought to obey

So in this sentence:

• debere is the main infinitive of the indirect statement
• parere is the infinitive that goes with debere

In other words:

• se ... debere = that they ought
• parere = to obey

Because pareo takes the dative case.

In English we say obey someone, using a direct object. But in Latin, pareo is constructed differently:

• alicui parere = to obey someone
• literally, something like to be obedient to someone

So:

• matri parere = to obey mother

with matri in the dative singular.

Latin often leaves out a possessive word when it is obvious from the context, especially with:

• family relationships
• body parts
• personal belongings closely connected to the subject

Here, since the subject is brother and sister, matri naturally means their mother.

Latin could say suae matri, but it does not have to. The possessive idea is understood.

Because noceo also takes the dative case.

So:

• alicui nocere = to harm someone
• more literally, to be harmful to someone

That is why Latin says:

• discordia omnibus nocet

with:

• discordia = subject, nominative
• omnibus = dative, to all / to everyone

Discordia is nominative singular, and it is the subject of nocet.

So in the clause:

• discordia omnibus nocet

the structure is:

• discordia = discord, disagreement
• omnibus = to everyone
• nocet = harms / is harmful

So discordia is the thing doing the harming.

Yes, Latin word order is more flexible than English word order because the endings show how the words function.

This sentence is arranged in a natural and clear way:

• Frater et soror = subject first
• tandem consentiunt = main action
• se matri parere debere = what they agree
• quia discordia omnibus nocet = reason

But other orders are possible, for example to emphasize different parts. The endings would still show the grammar.

So the order here is not the only possible one, but it is a very normal one.