Nimemkosea dada yangu, kwa hiyo naomba msamaha.

Nimemkosea can be broken into several parts:

• ni- = I
• -me- = a completed / present perfect idea, often like have
• -m- = him/her (object marker for a singular person)
• -kosea = wrong, do wrong to, offend

So Nimemkosea literally means something like I have wronged him/her or I have done wrong to him/her.

In this sentence, the person is then named explicitly as dada yangu = my sister.

That -m- is the object marker, and Swahili often uses it even when the noun is also stated.

So:

• Nimemkosea = I have wronged her
• dada yangu = my sister

Together, it is like saying:

I have wronged her, my sister.

In natural English we would usually just say I have wronged my sister, but in Swahili the object marker inside the verb is very normal.

This is a very common learner question.

• kosa usually means to make a mistake, to be wrong, or to miss/lack
• kosea often means to do wrong to someone, to offend someone, or to make a mistake in relation to someone/something

So:

• Nimekosa = I was wrong / I made a mistake
• Nimemkosea = I have wronged him/her

The ending -ea often adds the idea of doing something to, for, or in relation to someone/something.

In Swahili, possessives usually come after the noun.

So:

• dada yangu = my sister
• rafiki yangu = my friend
• kitabu changu = my book

This is the normal word order. English puts the possessive first (my sister), but Swahili usually puts it after the noun.

Kwa hiyo means therefore, so, for that reason, or as a result.

In this sentence:

• Nimemkosea dada yangu = I have wronged my sister
• kwa hiyo = therefore / so
• naomba msamaha = I ask for forgiveness / I apologize

So kwa hiyo links the two ideas clearly:
I have wronged my sister, so I am asking for forgiveness.

Both are possible, but naomba is very common and natural.

• naomba
• ninaomba

Both can mean I am asking or I ask, depending on context.

The shorter form without -ina- is extremely common in everyday Swahili. It is often preferred in normal speech unless you want extra emphasis or clarity.

So naomba msamaha is a very natural way to say I ask for forgiveness or I apologize.

Msamaha means forgiveness or pardon.

So:

• naomba msamaha = I ask for forgiveness
• in natural English, this often becomes I apologize

It is a polite and serious expression. It sounds a bit stronger and more explicit than a quick sorry.

Not exactly.

• samahani can mean sorry, excuse me, or pardon me
• naomba msamaha is more literally I ask for forgiveness

So samahani is broader and can be used in many situations, including small everyday ones:

• getting someone’s attention
• passing through a crowd
• apologizing lightly

But naomba msamaha sounds more direct and sincere when you want to admit wrongdoing and ask to be forgiven.

Because dada yangu refers to one person, and for a singular human object Swahili uses -m-.

This marker is not specifically about the word dada itself. It is about the fact that the object is:

• singular
• human

So -m- can mean him or her depending on context.

That is why Nimemkosea can mean:

• I have wronged him
• I have wronged her

The noun dada yangu tells you that here it means her / my sister.

Most literally, -me- gives a present perfect sense, so I have wronged my sister is very close.

But in real translation, English may use either:

• I have wronged my sister
• I wronged my sister

The best choice depends on the context and the style of English being used. Grammatically, though, the -me- form points to a completed action with present relevance, which is why have wronged is often the best match.

Yes, it is natural and clear.

It sounds like a thoughtful, deliberate statement:

Nimemkosea dada yangu, kwa hiyo naomba msamaha.

It is especially suitable when someone is admitting fault and asking forgiveness in a sincere way.

A speaker might also say similar things such as:

• Nimemkosea dada yangu. Naomba msamaha.
• Nimemkosea dada yangu, samahani.

But the original sentence is perfectly good and grammatically natural.