Usually children learn the words they need to know to get particular types of attention first, and use them most frequently.
So while there may be many reasons for her to choose saying "baba" and "dede" more frequently than "anne" one possibility is that you and grandpa don't pay as much attention to her as her mother. When she wants her mother's attention, she may already have it.
Mother might be anticipating her needs or paying attention more carefully to her other cues.
You and Grandpa, however, might not notice when she is trying to grab your attention unless she uses those words.
If this is the case - and clearly it's only one possibility of many - then if mom wants her to use "anne" to get mom's attention, she will have to let your daughter be a little more independent, stop anticipating her needs, and require her to ask for things she wants - including mom's attention.
In other words, it would be up to mom and the daughter to decide how best to communicate. If neither of them feel it necessary for the daughter to use "anne" then they won't.
This behavior actually speaks in favor of the closeness of their relationship, and probably shouldn't be viewed negatively.