That's because with younger sister, it says 真萌, which doesn't necessarily have to have to have the "le" at the end. However, when using 太, the phrase is incomplete unless you add the "le" after the adjective.
It's a measure word for animals, including cats, dogs and pandas. Trouble here is that the male voice uses the wrong tone falling-rising (3rd tone) which means "only". It should be a high tone (1st tone) if it is a classifier.
The 美图, 太萌 and 自拍 Word Bank tiles are silent, such tiles always stay silent for the entire lesson. Defective network code? Horrible experience, bye bye duolingo. Reported [The audio does not sound correct] 2019 June 23rd Sunday Opera 60.0.3255.170 Mac OS X 10.11.6 64-bit.
I wonder why this is a double lesson in one? Six parts on every level when the ones preceding where three parts on each level? It gives you a sense you'll be on it forever. Many complicated characters as well. No consistency in the course whatsoever.
There are 88 lessons in the Chinese tree, which is a significant number in Chinese culture (it's associated with becoming rich). I have a feeling that some content was shifted around to accomodate the 88 number.