Moja sestra je kupila dvije kifle i malo salame u pekari.

Because moja has to agree with sestra in gender, number, and case.

• sestra is feminine singular nominative
• so the possessive adjective must also be feminine singular nominative: moja

Compare:

• moj brat = my brother
• moja sestra = my sister
• moje dijete = my child

This is the normal Croatian past tense form.

It is built from:

• je = 3rd person singular of biti used as an auxiliary
• kupila = the past participle form of kupiti

So je kupila means bought.

A very important point: the participle agrees with the subject.

• on je kupio = he bought
• ona je kupila = she bought
• ono je kupilo = it bought

So here kupila shows that the subject is feminine singular: sestra.

Because je is a clitic, and Croatian clitics usually go in the second position in the sentence or after the first unit.

So:

• Moja sestra je kupila... = neutral, natural order

You will often see Croatian clitics behaving this way:

• Ivan je došao.
• Moja sestra je kupila kifle.

Not usually:

• Moja sestra kupila je... in neutral everyday prose

That version can appear, but it is much less neutral.

Because the number 2 changes form depending on gender.

• dva is used with masculine and neuter
• dvije is used with feminine

Since kifla is feminine, you say:

• dvije kifle

Compare:

• dva kruha / dva sendviča (masculine)
• dva jaja (neuter)
• dvije kifle (feminine)

Croatian uses a special counting pattern after 2, 3, and 4.

So you get:

• jedna kifla
• dvije kifle
• tri kifle
• četiri kifle
• pet kifli

For a learner, the most useful thing is simply to memorize the pattern.
With kifla, after dvije the form is kifle.

A helpful comparison:

• jedna kifla
• dvije kifle
• pet kifli

So the form changes again after 5 and above.

Because malo is a quantity word meaning a little or some, and after quantity words Croatian normally uses the genitive.

So:

• malo salame
• puno vode
• nešto kruha

Here salame is the genitive form of salama.

Also, in this sentence salama is being treated like a mass noun: not one whole salami as a countable object, but some amount of salami.

So:

• malo salame = a little / some salami
• dvije salame = two salamis

Because u can take different cases depending on meaning.

Here it means location: in / at the bakery, so Croatian uses the locative:

• u pekari

If it meant movement into the bakery, Croatian would use the accusative:

• Išla je u pekaru. = She went into/to the bakery.
• Bila je u pekari. = She was in/at the bakery.

So in your sentence, the buying happened there, which is why u pekari is used.

Yes. Very often Croatian uses the same phrase where English might choose either in or at depending on context.

So u pekari can naturally correspond to:

• in the bakery
• at the bakery

English chooses between those more strictly than Croatian does.

Because Croatian does not have articles like English.

So Croatian simply says:

• sestra
• kifle
• salame
• pekari

without separate words for a, an, or the.

If Croatian speakers want to be more specific, they can use other words, for example:

• ta pekara = that bakery
• ova sestra = this sister
• jedna kifla = one / a certain roll

But in ordinary sentences, no article is needed.

This is about aspect, which is very important in Croatian.

• kupiti = perfective → a completed act of buying
• kupovati = imperfective → buying in progress, repeated buying, habitual buying, or a more general process

In this sentence, the action is a single completed event, so kupiti is the natural choice:

• Moja sestra je kupila... = My sister bought...

If you said kupovala, it would suggest something more like:

• she was buying
• she used to buy
• she spent time buying

depending on context.

Yes. Croatian word order is more flexible than English word order, but different orders change the focus or emphasis.

The sentence you have is the most neutral:

• Moja sestra je kupila dvije kifle i malo salame u pekari.

Other orders are possible, for example:

• U pekari je moja sestra kupila dvije kifle i malo salame.
  Emphasizes where it happened.

• Dvije kifle i malo salame moja sestra je kupila u pekari.
  Emphasizes what she bought.

But the clitic je still has to follow normal clitic-position rules, so not every rearrangement is equally natural.

Not always in the most literal English sense.

With mass nouns, malo often corresponds to:

• a little
• some

So:

• malo salame can be translated as a little salami or simply some salami

This is one of those places where a very literal word-for-word English translation can sound less natural than the real meaning.