This can be very normal for 15 months, with the goal at one year as being able to say one word other than mama and dada and the goal at 15 months in being able to sat 3-7 words. Learning to talk is a combination of exposure and experience with speech. Repeating words for a few days and then moving on to something else is also normal.
To encourage speech the first step is to verbalize what you are doing during the day while she is in the room. Also be sure that you are reading books as often as she is interested and make it part of your pre-sleep routine early on. Point out what you are looking at and ask questions about things you are pretty sure she knows how to say. If there is a picture of a boy and a ball ask “What is he playing with?” If she does not answer, answer it for her and then see if she will repeat what you have said. “He has a ball. Can you say ball?” Another method is to ask her what she wants when she is pointing at something and you know what she is after. “Do you want milk? Say milk?” and then wait for her to say any sound in the word milk and then get very excited when she is successful with verbal praise and some milk.
At age 18 months she should say between 10-15 words, by age two this expands to 50 words and starting to put two word phrases together so between 18-24 months their vocabulary expands quickly. Even at age 2 only about half of what she says should be understandable by someone who hears her speak on a regular basis so you and daddy should understand her but a relative who only visits twice a year may not. It sounds like you are on the right track and although there are 18 month olds out there who can repeat the ABCs it does not mean their child is any more likely of getting into an Ivy league college than yours (or mine).