في الحفلة لا انا رقصت ولا اختي رقصت، بس كنا مبسوطين.

It works like neither ... nor ... in English.

So:

• لا أنا رقصت = I didn’t dance
• ولا اختي رقصت = and my sister didn’t dance / nor did my sister dance

Put together:

لا أنا رقصت ولا اختي رقصت
= Neither I danced nor my sister danced

This is a fixed negative pairing. It is different from the everyday Egyptian negation pattern ما...ش, which is used for ordinary single-clause negation.

Because in this pattern, the negative particle comes before the item or clause it is negating.

So Egyptian naturally says:

لا أنا ... ولا اختي ...

rather than something like:

أنا لا ...

That second type is not the normal everyday Egyptian way to say it here. In this sentence, لا is part of the neither...nor structure, so it comes first.

Because Arabic spelling usually does not write short vowels, so the same written form can represent different spoken forms.

Here, رقصت is understood differently from context:

• with أنا: رقصت = I danced
• with اختي: رقصت = she danced

In speech, they are not exactly the same. Roughly, you may hear something like:

• أنا رقصت = raʔaṣt
• اختي رقصت = raʔaṣit

The exact vowels can vary a bit by speaker, but the important point is: same spelling, different pronunciation and meaning depending on context.

Repeating the verb is very natural here and makes the neither...nor structure clear.

So this sounds normal:

لا أنا رقصت ولا اختي رقصت

Arabic can sometimes omit repeated words when they are obvious, but full repetition is common and clear.

If you wanted a different, more compact way to say the general idea, you could say:

أنا واختي ما رقصناش
= My sister and I didn’t dance

That is close in meaning, but it does not have the same explicit neither...nor structure.

Because Arabic usually omits the verb to be in the present, but it uses كان in the past.

Compare:

• إحنا مبسوطين = We are happy
• كنا مبسوطين = We were happy

So كنا is the past form meaning we were.

Because the verb كنا already tells you the subject is we.

In Arabic, subject pronouns are often left out when the verb already makes them clear.

So:

• كنا مبسوطين = We were happy
• إحنا كنا مبسوطين = also possible, but more emphatic

Adding إحنا can give a little extra emphasis or contrast, but it is not necessary.

Here, بس means but.

So:

بس كنا مبسوطين
= but we were happy

This is a very common Egyptian word. In other contexts, بس can also mean only, just, or even enough/stop, so the meaning depends on context.

Because it describes we, not one person.

The singular forms are:

• مبسوط = happy (masculine singular)
• مبسوطة = happy (feminine singular)

But with we, the sentence uses the plural:

• مبسوطين = happy (plural, in everyday Egyptian usage)

So كنا مبسوطين literally means we were happy.

In everyday Egyptian Arabic, مبسوطين is the normal, very common plural form in speech for groups, including mixed groups and often even groups of women.

So colloquial Egyptian often uses -ين very broadly in these predicate-adjective situations.

That means كنا مبسوطين sounds completely natural here.

It can absolutely have that wider meaning.

مبسوط in Egyptian can mean:

• happy
• glad
• pleased
• having a good time / enjoying oneself

So in this sentence, بس كنا مبسوطين can feel like:

• but we were happy
• but we were having a good time
• but we still enjoyed ourselves

Literally, في often means in, but with events it is very often best translated as at.

So:

في الحفلة
= at the party

This is very normal Arabic usage. Putting it at the beginning also helps set the scene first:

At the party, ...

Because in informal Arabic writing, especially texting and casual typing, people very often leave out the initial hamza.

So:

• أختي
• اختي

both mean my sister

The version without the hamza is extremely common in everyday informal writing.

Yes, but the sentence would be structured differently.

If you want the exact neither...nor feeling, لا ... ولا ... is a very good choice.

With ما...ش, a natural rewording would be:

في الحفلة أنا واختي ما رقصناش، بس كنا مبسوطين.

That means:

At the party, my sister and I didn’t dance, but we were happy.

This is very close in meaning, but it focuses on the two of you together, rather than using the explicit neither I... nor my sister... pattern.