The possession suffix indicates that one thing is owned by another. Below are its forms:
||lErİ / s(İ)
Below you can see some examples to see how the suffix is used.
|her elder sisters
||the child's father
|their elder sister
|their elder sisters
Note that personal pronouns in front of the possessed object are always optional, since the suffix of the possessed object already indicates which person is the owner.
Also note that her elder sisters, their elder sister and their elder sisters can all be translated by ablaları.
You can disambiguate her elder sisters by saying onun ablaları.
You can disambiguate their elder sister by saying onların ablası, using the third person singular suffix.
Now you might think that you can disambiguate their elder sisters by saying onların ablalarları, with one lar for indicating that the possessor is plural, and the other lar for indicating that the possessed object is plural.
Unfortunately, you can never use the plural suffix twice in a single word, so this is not possible.