Learn Hardcore Finnish: Siskoni pelaa koripalloa samassa joukkueessa ystävänsä kanssa. - My sister plays basketball on the same team as her friend.

Using minun siskoni is possible and grammatically correct, but it’s more emphatic, like saying my sister (as opposed to someone else’s).
In a neutral sentence, Finnish usually prefers just the noun + possessive suffix: siskoni pelaa…

What does the -ni in siskoni mean exactly?

The suffix -ni is a possessive suffix meaning my. It attaches to the noun:

sisko (sister) → siskoni (my sister)
talo (house) → taloni (my house)

So possessive pronouns (minun, sinun, hänen, etc.) are often not needed, because the suffix already tells you who something belongs to.

Why is koripalloa in the partitive case and not koripallon or just koripallo?

Koripalloa is the partitive singular of koripallo.

With the verb pelata (to play), the game or sport is very often in the partitive, because you’re doing an ongoing activity rather than affecting a clearly bounded object. It’s similar to saying:

He plays basketball (an activity)
not
He plays the basketball (a specific ball)

So:

pelata koripalloa = to play basketball (the sport, in general)

You could see koripalloa as “some basketball / basketball (as an activity)”.

What case is samassa joukkueessa, and what does it literally mean?

Both samassa and joukkueessa are in the inessive case (the “in” case):

sama → samassa = in the same
joukkue → joukkueessa = in (the) team

Together:

samassa joukkueessa = in the same team

Finnish doesn’t use separate prepositions like in here; the -ssa / -ssä ending itself carries the meaning in.

How is joukkueessa formed from joukkue?

The base form is joukkue (team). To form the inessive (in), you:

Take the stem joukkue-
Add the inessive ending -ssa

→ joukkueessa = in the team

There is no consonant gradation here; the main change is just +ssa.

Why is it samassa joukkueessa, not sama joukkueessa?

The adjective sama (same) has to agree in case with the noun it describes.

Nominative: sama joukkue (the same team – as subject)
Inessive: samassa joukkueessa (in the same team)

Since the noun joukkue is in the inessive (joukkueessa), sama must also be in the inessive: samassa.

What does ystävänsä mean, and what is the -nsä ending?

Ystävänsä breaks down as:

ystävä = friend
ystävän = friend’s (genitive singular)
-nsä = his / her / their (own) (3rd person possessive suffix)

So ystävänsä means his/her/their (own) friend’s (in the sense “of their friend”). In this sentence, it refers back to the subject:

Siskoni … ystävänsä kanssa.
→ My sister … with her (own) friend.

The -nsä suffix indicates that the friend belongs to the same person who is the subject (here, the sister).

Why is it ystävänsä kanssa and not just ystävä kanssa?

Two things are going on:

kanssa (“with”) requires the preceding noun to be in the genitive case:
- ystävä (friend) → ystävän (friend’s) before kanssa
- ystävän kanssa = with (a/the) friend
The suffix -nsä adds the meaning his/her/their (own):
- ystävä
  - n
    - nsä → ystävänsä

So ystävänsä kanssa literally is “with his/her/their (own) friend’s”, but functionally means with his/her/their (own) friend.

Just ystävä kanssa would be incorrect; you must have the genitive before kanssa: ystävän kanssa.

What is the difference between ystävänsä kanssa and ystävän kanssa?

ystävän kanssa
- a/the friend, but no indication of whose friend
- could be anyone’s friend
ystävänsä kanssa
- his/her/their (own) friend
- the possessive suffix -nsä links the friend back to the subject (here, the sister)

So:

Siskoni pelaa koripalloa samassa joukkueessa ystävän kanssa.
→ My sister plays basketball in the same team with a/the friend. (ambiguous whose friend)
Siskoni pelaa koripalloa samassa joukkueessa ystävänsä kanssa.
→ My sister plays basketball in the same team as her own friend.

Is kanssa a preposition or a postposition, and how does it work?

Kanssa is a postposition:

It comes after the noun phrase.
The noun before it is in the genitive case.

Structure:

ystävä (friend) → ystävän (friend’s) + kanssa (with)
ystävän kanssa = with (a/the) friend

In the sentence:

ystävänsä kanssa = with her friend
(literally: “with her friend’s”)

Could the word order be Siskoni pelaa ystävänsä kanssa samassa joukkueessa? Would it change the meaning?

Yes, that word order is also possible:

Siskoni pelaa ystävänsä kanssa samassa joukkueessa.

The basic meaning stays the same: My sister plays basketball on the same team as her friend.

Differences:

Finnish word order is relatively flexible and is often used to highlight different parts.
Putting ystävänsä kanssa earlier can slightly emphasize with her friend, but in a neutral context both orders are natural and mean essentially the same thing.

How would the sentence change if I wanted to emphasize that it’s someone else’s friend, not the sister’s own friend?

To avoid the idea of her own friend, you would usually avoid the possessive suffix -nsä and specify whose friend it is:

Examples:

Siskoni pelaa koripalloa samassa joukkueessa ystävän kanssa.
→ with a/the friend (ambiguous whose friend)
Siskoni pelaa koripalloa samassa joukkueessa hänen ystävänsä kanssa.
→ with his/her friend (referring to some other previously mentioned person, not the sister)
Siskoni pelaa koripalloa samassa joukkueessa veljeni ystävän kanssa.
→ with my brother’s friend

So the possessive suffix -nsä normally ties the friend to the subject; using a separate possessive pronoun or another noun (like veljeni) can redirect that possession to someone else.