Mój kuzyn mieszka w tym samym mieście co moja kuzynka.

In Polish, possessive adjectives (like mój / moja / moje) must agree in gender, number, and case with the noun they describe.

• kuzyn is masculine singular → it takes mój
  
  • mój kuzyn = my (male) cousin
• kuzynka is feminine singular → it takes moja
  
  • moja kuzynka = my (female) cousin

Basic forms of mój in the nominative singular:

• mój – with masculine nouns (e.g. mój brat, mój kuzyn)
• moja – with feminine nouns (e.g. moja siostra, moja kuzynka)
• moje – with neuter nouns (e.g. moje miasto, moje dziecko)

So the difference is purely grammatical agreement with the gender of kuzyn vs kuzynka.

Yes:

• kuzyn = male cousin
• kuzynka = female cousin

These are the most common and neutral words for cousin in modern Polish.

You may also see:

• brat cioteczny – literally “aunt/uncle-related brother” (male cousin)
• siostra cioteczna – literally “aunt/uncle-related sister” (female cousin)

These are more formal or old-fashioned in everyday speech, and they also imply a side of the family (maternal/paternal) if used precisely. In normal conversation, Poles simply say kuzyn and kuzynka.

Two things are going on:

1. The preposition w (in) for location requires the locative case, not nominative or accusative.

   • miasto (nominative) → w mieście (locative)

2. The phrase ten sam (the same) is declined to match miasto in gender, number, and case.

For miasto (neuter, singular, locative):

• to samo miasto – nominative/accusative (e.g. to jest to samo miasto – this is the same city)
• w tym samym mieście – locative (because of w)

So:

• w
  • tym samym
    • mieście
      = in the same city (correct, with locative)
• w to samo miasto would be ungrammatical for a static location.

Miasto is a neuter noun. In the locative singular, many neuter nouns change:

• the ending -o → -e
• often with a consonant or vowel change in the stem

For miasto:

• Nominative: miasto (a city)
• Locative: mieście (in the city, about the city)

This type of change (often -asto → -eście, -sto → -ście) also appears in words like:

• ciasto → w cieście (in the dough/cake)

So w mieście is simply the correct locative form required after w when talking about location.

With ten sam / ta sama / to samo (the same), Polish normally uses co in such comparisons:

• ten sam człowiek co wtedy – the same person as then
• ta sama szkoła co rok temu – the same school as last year
• w tym samym mieście co moja kuzynka – in the same city as my cousin

Compare:

• ten sam … co … – the same … as …
• taki sam … jak … – the same kind / just as … as …

So you say:

• ten sam dom co wcześniej
  but
• taki sam dom jak wcześniej

Który would make it more like a relative clause:

• w tym samym mieście, w którym mieszka moja kuzynka
  = in the same city in which my cousin lives

That is correct but longer and more formal. The original co is the natural, everyday comparative construction with ten sam.

In Polish, after co (or jak) in this type of comparison, you normally keep the nominative form, as if you were just naming the other person/thing in parallel:

• taki wysoki jak ja – as tall as I (not jak mnie)
• ten sam nauczyciel co ty – the same teacher as you
• w tym samym mieście co moja kuzynka – in the same city as my cousin

So moja kuzynka stays nominative, just like it would in a simple sentence Moja kuzynka mieszka w tym mieście. Formally, you can think of it as a shortened form of:

• … w tym samym mieście, co (w nim) mieszka moja kuzynka.

Polish word order is flexible, but not all permutations sound natural.

Possible and natural:

• Mój kuzyn mieszka w tym samym mieście co moja kuzynka.
• W tym samym mieście mieszka mój kuzyn co moja kuzynka.
  (sounds a bit more marked/emphatic, focusing on the city)

Your variant:

• Mój kuzyn w tym samym mieście mieszka co moja kuzynka.

is understandable but sounds awkward and non‑native. Typically, the verb mieszka stays close to its prepositional phrase w tym samym mieście, and the comparison co moja kuzynka comes at the end.

Grammatically, yes, but in practice it sounds odd and potentially confusing.

Polish often drops possessive pronouns when context is very clear:

• Idę do mamy. – I’m going to (my) mom.
• Dzwonił brat. – (My) brother called.

Here, however, kuzyn and kuzynka without mój / moja could sound like you’re talking about some cousin and some female cousin, not necessarily both yours, and it’s not a strongly established “family member = automatically mine” context.

So:

• Mój kuzyn mieszka w tym samym mieście co moja kuzynka. – best and clearest.
• Kuzyn mieszka w tym samym mieście co kuzynka. – technically possible, but stylistically odd unless the larger context already makes ownership absolutely obvious.

In this specific structure, ten sam / w tym samym … co …, standard punctuation does not use a comma:

• To jest ten sam człowiek co wczoraj.
• Mieszkają w tym samym domu co my.
• Mój kuzyn mieszka w tym samym mieście co moja kuzynka.

A comma appears when co clearly introduces a full subordinate clause:

• To jest to, co lubię. – This is what I like.
• Powiedz mi, co zrobiłeś.

Here co is part of a comparative phrase with tym samym, not a separate clause, so no comma is needed. Some speakers may insert one in informal writing, but it’s not standard.

Polish distinguishes between:

• mieszkać – to live somewhere in the sense of reside, have one’s home
• żyć – to live in the sense of be alive, live one’s life

So for an address or place of residence, you use mieszkać:

• Mój kuzyn mieszka w Warszawie. – My cousin lives (resides) in Warsaw.
• Ona mieszka w tym samym mieście.

Żyć is used differently:

• On już nie żyje. – He is no longer alive.
• Chcę żyć inaczej. – I want to live differently.

That’s why mieszka is correct in Mój kuzyn mieszka w tym samym mieście co moja kuzynka.