بين ما السباح عم يسبح، اخته بتستناه عند حافة الحوض.

It means while or at the same time that.

In this sentence, بين ما introduces an action that is happening in the background:

• بين ما السباح عم يسبح = while the swimmer is swimming

A good thing to remember: don't translate بين ما word-for-word. As a chunk, it simply means while.

Also, in more formal Arabic, you often see بينما as one word. In informal Levantine writing, بين ما as two words is very common.

عم is a very common Levantine marker for an action that is in progress.

So:

• يسبح = he swims / he is swimming
• عم يسبح = he is swimming, specifically emphasizing that the action is happening right now

That is why عم يسبح fits well after بين ما: it shows an ongoing action during which something else is happening.

In Levantine, speakers do not always need عم on every present-time verb.

Here:

• عم يسبح strongly marks the swimmer's action as ongoing
• بتستناه can still naturally mean she is waiting for him, especially because the context already makes the timing clear

So the sentence is understood as two simultaneous actions even though only one verb has عم.

You could hear عم تستناه in some situations too, but بتستناه is perfectly natural here.

بتستناه can be broken down like this:

• بـ = present/non-past marker in Levantine
• تستنى = the verb form for she waits
• ـه = him

So بتستناه means:

• she waits for him
• or in this context, she is waiting for him

The verb is based on استنى / يستنى, which means to wait or to wait for.

The attached ـه is a pronoun, but its job depends on what it attaches to.

In بتستناه

• ـه = him
• It is the object of the verb
• So بتستناه = she is waiting for him

In أخته

• ـه = his
• It shows possession
• So أخته = his sister

So the same attached pronoun can mean:

• him when attached to a verb
• his when attached to a noun

Yes. In informal Arabic writing, especially online or in casual messages, people often leave out the hamza in spelling.

So:

• أخته = more careful spelling
• اخته = very common casual spelling

They both mean his sister.

This is normal in colloquial writing and does not change the meaning.

Here, عند means at or by.

So:

• عند حافة الحوض = at the edge of the pool

This is worth noticing because learners often first meet عند in meanings like:

• with
• have (as in عندي = I have)

But عند can also mark location, especially something like by/at a place.

Because this is an iḍāfa construction, the Arabic way of saying things like:

• the edge of the pool
• the door of the house
• the teacher's book

In an iḍāfa:

• the first noun usually does not take ال
• the second noun shows whether the whole phrase is definite

So:

• حافة = edge
• الحوض = the pool
• حافة الحوض = the edge of the pool

Even though حافة has no ال, the whole phrase is definite because الحوض is definite.

Literally, حوض can mean something like a basin, tank, or pool.

In this sentence, because of the context with السباح and يسبح, it clearly means the pool, specifically a swimming pool.

So the meaning is natural even though the word itself can be used in a few related senses depending on context.

Because Arabic verbs already contain subject information.

For example:

• يسبح already tells you the subject is he
• بتستناه already tells you the subject is she

And since the sentence already has nouns:

• السباح = the swimmer
• أخته = his sister

there is no need to add separate pronouns like هو or هي.

Arabic often leaves those separate pronouns out unless there is special emphasis or contrast.

It is mostly Levantine in grammar, but some of the vocabulary is fairly neutral and can sound a bit more formal or general.

Colloquial Levantine features

• عم يسبح
• بتستناه
• casual spelling like اخته

More neutral/formal-looking vocabulary

• السباح
• حافة
• الحوض

So this kind of sentence is very normal for teaching materials: colloquial structure, with vocabulary that is easy to understand across varieties of Arabic.