Aunts, uncles, nieces, nephews, cousins, etc. usually represent an aspect of yourself relative to the character of the person in your dream. Sometimes represents your family traditions, subconscious attitudes, conventions, or even talents, that are part of the unique psychological and social environment a family provides.

Baby - A baby generally represents you feelings such as possessiveness, insecurity, joy, curiosity, vulnerability, lack of responsibility, a need for love, intense dependence, innocence, and are symbols of the purest form of a human whose possibilities are endless. Also represents a new phase of life.
Many people from time to time will have babies or small children in their dreams. If the baby is a stranger to you, you can assume that it represents you. You are the baby and the dream is telling you something about your development in a particular area of your life. At times of great change and renewal, a baby may appear in a dream and represent your potential and a new beginning.

Twin(s) - signifies ambivalence, dualities and opposites, and the balance in yourself that is extremely important to your emotional and psychological health.

Boy - If you are female, and you dream on an unknown boy, then it indicates that you are developing the masculine aspects of character. Alternatively, it may represent your feelings about a real-life boy who is important and significant to you, and your motherly instincts may be taking over.
If you are a male and dream of an unknown boy, then it suggests your playful, innocent, childlike nature. Alternatively, it can symbolize the immature aspects of yourself that still needs to grow. Your inner child may be trying to draw your attention to parts of yourself that you need to recognize and acknowledge.

Girl - For a female to see a girl, and if she reminds you of someone you know, it's a representation of an aspect of that person you recognize about yourself, or would like to.
If you do not recognize her, or she does not remind you of anyone you know, she would then represent your playful, innocent, and childlike nature. Perhaps you have been behaving 'girlish', or immaturely, about something or situation. It could also be your maternal side taking over.
For a man to see an unknown girl, signifies that he is becoming aware of, or recognizing and acknowledging, his younger, feminine, side.
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Has there been a big change in your life recently, or is there one approaching?
Have you been feeling vulnerable, or dependent?
Is there a decision that you need to make, and you're still undecided about it?
Are you stuck between what you should do, and what you want to do?

Hope I've helped, good luck!