What's wrong with my sister ?

Waking up dizzy in the morning could be a sign that you have low blood sugar, so you're dizzy before you eat any food in the morning.

Or it could be an inner ear infection called labyrinthitis, which can affect your balance and make you feel dizzy when waking up in the morning. That was my first thought. Even built up ear wax could be the problem. Or getting up too fast in the morning, or meditations.

If shes waking up dizzy in the summer only, it sounds like she's not drinking enough water. Dehydration is another cause of morning dizziness. This can happen as a result of a condition such as hyperglycemia (high blood sugar). Have they checked for a hormonal imbalance? She needs to maintain a healthy lifestyle, including eating a balanced diet and drinking plenty of water (at least 64 ounces) per day. Also, no skipping meals. Her eating schedule could be off, or maybe she is overexcercising. Are there any other symptoms besides the dizziness? Any pain, double vision, rapid heartbeat, severe headache, or vomiting? I'd be getting another doctor's opinion.

Dehydration… she needs to drink more water.

.. especially when waking up. I often lay in bed for a few minutes drinking cold water upon waking. Also when you wake in the morning it’s important to give yourself time to wake up… no jumping up and showering or running around. Wake up. Have some water. Use the washroom. Sit for a few minutes then start the day and always while having some fluids.. juice tea or water or coffeE. Hope she feels better.