Learn Hardcore Swahili: Siyo tu mimi ninapenda kusoma, bali pia dada yangu anafurahia kusoma riwaya ndefu. - Not only do I like reading, but my sister also enjoys reading long novels.

Siyo tu mimi ninapenda kusoma, bali pia dada yangu anafurahia kusoma riwaya ndefu.
→ Not only do I like reading, but my sister also enjoys reading long novels.

You can use the same pattern with other subjects or verbs:

Siyo tu wanafunzi wanajifunza, bali pia mwalimu anajifunza.
Not only are the students learning, but the teacher is also learning.

Why is it siyo tu mimi and not si mimi tu? Are both possible?

Both structures are possible, but they sound slightly different in emphasis:

Siyo tu mimi ninapenda kusoma...
- This matches the more “formulaic” pattern siyo tu ... bali pia ....
- The focus feels more on the whole idea “not only I, but also my sister”.
Si mimi tu ninayependa kusoma, bali pia dada yangu...
- Literally: It’s not only me who likes reading, but also my sister...
- Here the emphasis feels more strongly on mimi (“me”) as the contrast point.

Your sentence is using the more idiomatic expression siyo tu ... bali pia ....
Si mimi tu ... is grammatical and natural, but it slightly shifts the focus.

Why do we have both mimi and ni- in mimi ninapenda? Isn’t that redundant?

Yes, in a sense it is redundant, because:

ni- in ninapenda already means “I”.
mimi also means “I / me”.

However, using both is very common and natural for emphasis or clarity:

Ninapenda kusoma. – I like reading. (neutral)
Mimi ninapenda kusoma. – I (for my part) like reading.
(emphasizing the I, contrasting with others)

In your sentence, mimi helps set up the contrast with dada yangu (“my sister”) in the second clause.
You could say Siyo tu ninapenda kusoma... and it would still be correct, just a bit less explicitly contrasted.

Can ninapenda be shortened to napenda here?

Yes:

ninapenda and napenda are both correct in the present tense.

ni- is the subject prefix for “I”. In casual speech and writing, speakers sometimes drop the first vowel (especially with ni- and u-), so you’ll hear:

Ninapenda kusoma. → Napenda kusoma.

Both are acceptable; ninapenda is a bit more careful/standard.
In your sentence, you could say:

Siyo tu mimi napenda kusoma, bali pia dada yangu anafurahia...

and it would still sound natural.

Could I use lakini pia instead of bali pia?

You can, but there is a nuance:

lakini = but / however (general contrast, often opposing ideas)
bali = but rather / but instead / but also (correcting or “adding on” in a special way)

In the fixed pattern siyo tu ... bali pia ..., bali pia is the most idiomatic choice.

Compare:

Siyo tu mimi ninapenda kusoma, bali pia dada yangu...
Not only I, but also my sister... (classic pattern)

If you say:

Siyo tu mimi ninapenda kusoma, lakini pia dada yangu...

it is understandable, and some speakers do say it, but bali pia sounds more “textbook correct” and stylistically smoother in this exact “not only ... but also ...” structure.

Why is kusoma repeated in the second part? Could I say ... bali pia dada yangu anafurahia riwaya ndefu?

You can drop the second kusoma, but the meaning shifts slightly:

... dada yangu anafurahia kusoma riwaya ndefu.
- My sister enjoys *reading long novels.*
- The verb anafurahia directly takes kusoma riwaya ndefu as its object.
... dada yangu anafurahia riwaya ndefu.
- My sister enjoys long novels.
- Now anafurahia takes riwaya ndefu as its object.
- The idea of reading is only implied, not explicitly stated.

In your original sentence, repeating kusoma makes it very clear that both people enjoy reading, not just that they like novels in some general way.

What is the difference between anapenda and anafurahia for “she enjoys/likes”?

Both relate to liking something, but:

anapenda – she likes / loves (general liking or affection)
anafurahia – she enjoys / takes pleasure in (more about the feeling of joy)

Subtle nuance:

Dada yangu anapenda kusoma riwaya ndefu.
My sister likes reading long novels. (a general preference)
Dada yangu anafurahia kusoma riwaya ndefu.
My sister enjoys reading long novels. (highlights the enjoyment/satisfaction during the activity)

Your sentence chooses anafurahia to emphasize that reading long novels is enjoyable for her, not just something she “likes” in an abstract way.

Why is it dada yangu and not dada wangu or something like dada ya mimi?

This is about noun class and possessive agreement.

dada belongs to the N-class (like ndugu, nyumba, habari).
The possessive “my” is formed with a class marker + -angu.

For the N-class, the possessive form is:

yangu → dada yangu = my sister

Other examples with N-class nouns:

nyumba yangu – my house
habari yangu – my news
rafiki yangu – my friend

wangu is used with M-/WA-class people nouns:

mtoto wangu – my child
mwalimu wangu – my teacher

dada ya mimi is not correct in standard Swahili; you should say dada yangu.

Why is the adjective ndefu used with riwaya? How does agreement work in riwaya ndefu?

Ndefu is the adjective form of -refu (long) agreeing with an N-class noun.

riwaya is N-class.
Adjectives must agree with the noun class.

For -refu:

M-/WA-class: mtu mrefu, watu warefu – tall/long person, people
KI-/VI-class: kitabu kirefu, vitabu virefu – long book(s)
N-class: often ndefu (with an n- plus some sound changes)

So:

riwaya ndefu – long novel
hadithi ndefu – long story
nywele ndefu – long hair

That’s why it’s riwaya ndefu, not riwaya refu.

Can I change the word order in the second part, e.g. bali dada yangu pia anafurahia...?

Yes, you can move pia around for slightly different emphasis, and all are acceptable:

... bali pia dada yangu anafurahia kusoma riwaya ndefu.
(neutral; pia is tied to the whole noun phrase dada yangu)
... bali dada yangu pia anafurahia kusoma riwaya ndefu.
(emphasizes that she, too, enjoys it)
... bali dada yangu anafurahia pia kusoma riwaya ndefu.
(can sound like “my sister also enjoys reading long novels” among other activities she enjoys)

Your original bali pia dada yangu... fits very naturally with the “not only ... but also ...” pattern, where pia marks the parallel addition.

What tense/aspect is anafurahia, and how would the meaning change with amefurahia or atafurahia?

Anafurahia is in the present habitual / present progressive:

ana- = present tense marker for “he/she”
-furahia = enjoy

So:

anafurahia = she enjoys / she is enjoying

If you change it:

amefurahia – she has enjoyed / she has been pleased (by it)
(completed action or present result)
atafurahia – she will enjoy
(future)

In your sentence, anafurahia matches ninapenda (both present), expressing a general, ongoing habit or tendency: both of you currently and generally like/enjoy reading.