صاحب اختي كان عم ياخد صور عند النهر لما طلعت الشمس.

It can mean either, depending on context.

In everyday Levantine, صاحب often suggests boyfriend when talking about relationships, but its basic sense is friend / companion. If you specifically meant husband, you would usually say جوز أختي or زوج أختي instead.

Because صاحب اختي is an iḍāfa construction, often called a possessive construction.

• صاحب = friend / boyfriend
• اختي = my sister

Together, they mean the friend/boyfriend of my sister. In this structure, the first noun usually does not take الـ, even though the whole phrase is definite.

In informal Levantine writing, people often leave out the hamza in spelling. So اختي and أختي both represent my sister.

In speech, it is usually pronounced something like ekhti in Levantine.

It is a past progressive form, meaning was taking.

• كان = was
• عم = marks an ongoing action
• ياخد = he takes / take

So كان عم ياخد صور means he was taking pictures.

In Levantine, عم is a very common way to show that an action is in progress.

So:

• ياخد صور = he takes / take pictures
• عم ياخد صور = he is taking pictures
• كان عم ياخد صور = he was taking pictures

It helps show that this was an ongoing action in the past, not just a single completed event.

After كان عم, Levantine commonly uses the imperfect verb without بـ.

So كان عم ياخد is very natural.
You may also hear كان عم بياخد in some speech, but كان عم ياخد is extremely common and standard for Levantine conversation.

Yes. ياخد is the Levantine colloquial form of MSA يأخذ.

A lot of colloquial verbs are simplified compared with MSA, both in pronunciation and spelling. So a learner who knows أخذ / يأخذ should recognize ياخد as the same verb in dialect.

Because ياخد صور is the natural way to say take pictures / take photos as an activity.

If you wanted to refer to just one photo, you would say ياخد صورة. Using the plural sounds more natural when describing someone photographing in general.

Here عند means at / by / near.

So عند النهر means by the river or at the river.
If you said في النهر, that would mean in the river, which is a different idea.

لما means when here.

It introduces the time clause: when the sun came up. In this sentence, it sets up the event that happened while the other action was already in progress.

So the structure is roughly:

• ongoing background action: كان عم ياخد صور
• event that happened during it: لما طلعت الشمس

Because الشمس is grammatically feminine in Arabic.

That means the past-tense verb must also be feminine:

• طلع = he/it came up
• طلعت = she/it came up

So with الشمس, you say طلعت الشمس.

Because verb-first order is very common in Arabic, especially in narration.

So طلعت الشمس sounds very natural as the sun came up.
الشمس طلعت is also possible, but it gives a bit more emphasis to the sun itself.

A good approximate pronunciation is:

ṣāḥeb ekhti kān ʿam yākhod ṣowar ʿand in-nahr lammā ṭelʿet ish-shams

There is some regional variation, but this is a solid Levantine-style reading for a learner.