Is there any particularly situation where it would be better to use the -ler or drop the -ler?
would this alternate suffix be more for consonant harmony or just personal preference?
kardeş - brother (or other social equal*) abi is older brother kız kardeş - younger sister (abla is older sister)
*kardeş is also used as a respect, when you don't know the person's age but want to express the bond/respect. (if known older, abi/abla). in speech i find names or the relevant "my brother/ sister" form are frequently used.