Learn Hardcore Polish: Wczoraj poszłam z siostrą do kawiarni i długo rozmawiałyśmy. - Yesterday I went to a café with my sister and we talked for a long time.

There is no form poszedłam in standard Polish; that mix is a common learner error. You simply have to memorize poszedłem (m.) and poszłam (f.) as the correct 1st person singular past forms.

Who can use poszłam? Does it depend on me or on siostra?

Poszłam depends on the speaker’s gender, not on siostra.

If the speaker is female: Wczoraj poszłam…
If the speaker is male: Wczoraj poszedłem…

The fact that you went with your sister (z siostrą) does not affect the ending of poszłam. It only describes who accompanied you, not who the subject is.

Why is it rozmawiałyśmy and not rozmawialiśmy?

Both are 1st person plural past tense, but:

rozmawiałyśmy – we (all female) were talking
rozmawialiśmy – we (at least one male) were talking

In the sentence, the subject is we = I + my sister, so it is a group of only women. That requires the feminine plural ending -łyśmy.

Why is there no my (we) in …i długo rozmawiałyśmy?

Polish usually drops subject pronouns (ja, ty, on, my, wy, oni) because the verb ending already shows the person (and, in the past, also gender):

rozmawiałyśmy clearly means we (female) were talking

You can say My długo rozmawiałyśmy, but it sounds more emphatic, like We really talked for a long time. The neutral everyday sentence just omits my.

Why is it z siostrą and not z siostra or z siostrę?

The preposition z (with) normally takes the instrumental case.

Nominative (dictionary form): siostra (sister)
Instrumental singular: siostrą (with accent on the first syllable)

So after z you must use siostrą:
z siostrą = with (my) sister.

Forms like z siostra or z siostrę are ungrammatical.

Why is it do kawiarni instead of do kawiarnia or do kawiarnię?

The preposition do (to, into) takes the genitive case.

Nominative: kawiarnia (café)
Genitive singular: kawiarni

So you must say:

do kawiarni = to the café

Do kawiarnia (nominative) and do kawiarnię (accusative) are wrong because they don’t match the case required by do.

What is the difference between do kawiarni and w kawiarni?

do kawiarni – movement towards the café: to the café
w kawiarni – location inside the café: in the café

Compare:

Wczoraj poszłam z siostrą do kawiarni – Yesterday I went to the café with my sister.
Wczoraj byłam z siostrą w kawiarni – Yesterday I was in the café with my sister.

The first focuses on going there, the second on being there.

Why is it długo and not długa or długą?

Długo is an adverb meaning for a long time.

długo rozmawiałyśmy – we talked for a long time

Forms like:

długa – feminine adjective (long)
długą – feminine adjective (accusative/instrumental)

Those are used to describe nouns, e.g.:

długa rozmowa – a long conversation
Miałyśmy długą rozmowę – We had a long conversation.

But in your sentence you are describing how you talked, so you need an adverb: długo.

Is the word order Wczoraj poszłam z siostrą do kawiarni fixed? Could I say Poszłam wczoraj…?

The word order is flexible. Both are correct:

Wczoraj poszłam z siostrą do kawiarni…
Poszłam wczoraj z siostrą do kawiarni…

Starting with Wczoraj is very common when narrating past events and slightly emphasizes when it happened. Moving wczoraj after the verb is also natural and doesn’t change the basic meaning. Polish relies more on endings than on strict word order.

Why is poszłam (perfective) used, but rozmawiałyśmy is imperfective? Does that contrast mean something?

Yes, it shows aspect contrast:

poszłam – perfective (from pójść): the act of going is seen as a single, completed event.
rozmawiałyśmy – imperfective (from rozmawiać): the talking is seen as an ongoing activity, here with duration (długo).

So the sentence structure is:

I went (completed) to the café,
and we were talking / talked (activity, lasting for some time) for a long time.

This perfective + imperfective combination is very typical in Polish narration.

How would the sentence change if the speaker were male?

You would change the past tense forms to masculine:

Wczoraj poszedłem z siostrą do kawiarni i długo rozmawialiśmy.

Changes:

poszedłem – I went (male speaker)
rozmawialiśmy – we talked (at least one male in the group: you + sister)

Could I say Wczoraj byłam z siostrą w kawiarni instead of Wczoraj poszłam z siostrą do kawiarni? Is there a difference?

Yes, you can, but there is a nuance:

Wczoraj poszłam z siostrą do kawiarni – emphasizes the going there, the movement.
Wczoraj byłam z siostrą w kawiarni – emphasizes the fact of being there, your presence in the café.

Both are natural; you choose depending on what you want to highlight: the action of going vs. the state of being there.

How would the part z siostrą change if I went with my sisters (plural)?

You need the instrumental plural of siostra:

Nominative plural: siostry (sisters)
Instrumental plural: siostrami

So you would say:

Wczoraj poszłam z siostrami do kawiarni i długo rozmawiałyśmy.

(If you are female.)
If you are male:

Wczoraj poszedłem z siostrami do kawiarni i długo rozmawialiśmy.