I have this sentence: Ben yarın sabah annemlere kahvaltıya gideceğim.

I was told this meant: I am going to Mother´s with (or and) others for breakfast tomorrow.

I was surprised with the ler in annemlere. Any explanation or link would be great.

-ler/lar  as in different usages

-ler/lar  may not always refer plurality. It may be used as :

a)     a family name /title  when  refer a certain family, we can use –ler/lar adding onto a persons name.  It refer a family which is actually “one family”

Bugün  Haticeler gelecek.  -à Today Hatices will come. [ Hatice’s  family will come today ]

b)    Name of  a civilization which actually refer a single civilization.

Bu  Urartular’dan kalma bir kale.

This is a castle [remained] from Urartus.

c)     Showing respect

Müdür Beyler henüz gelmediler. -à Mr. Manager hasn’t come yet.

d)    Mocking, underestimating

Hanımefendiler  daha uyanmadılar !  -àThe lady  hasn’t got up yet !

e)     Approximation [about, around]

6 yaşlarında bir çocuktu.

He/She was a child aged around 6 years.

f)      Exaggeration

Ateşler içinde kıvranıyordu.

He/She was writhing [suffering a lot] because of  fevers.

So, annemler in that context refers "my mother" [my mother´s house/my parents] 

I am going to my mother [my mother´s house] for breakfast tomorrow.

Thankyou.

I was told in this sentence annemler was representing: my mother and my sisters. I guess like saying mother and familiy.

Is that possible or is that not correct?

Actually it represents the house where mother/father  is residing. Say you were living with your parents and you got married someone and left home, you can call mother´s house "annemler /babamlar " .  Of course your single sisters or brothers may still live in your mother´s house.

Sisters/Brothers who are still residing at mother´s house won´t stop you to call that house as "annemler"