1. I have an older brother and little sister
2. My older brother is fat. Little sister is not fat 
3. Older brother really likes drinking coke. Little sister really likes eating apples and drinking water. 
4. I like drinking coke. I don't want to be fat. 
5. Older brother is drinking coke, little sister is drinking water. 

correct

I have a big brother and a younger sister. 
my younger brother  is fat. 

my younger sister not fat

younger brother likes cola

younger sister likes apple and water 

i like cola i don't want to be fat. 

younger brother is drinking cola. 

cola wo he mei mei zia water
Younger sister and I are drinking water. 

I have one older brother and one younger sister. My older brother is very fat. My younger sister is not fat. My brother(older) really likes to drink cola. My sister (younger) really likes to eat apples and drink water. I like to drink cola, I am not fat. My brother will drink cola, my sister will drink water 

I have an older brother and younger sister. My older brother is fat. My younger sister isn't fat. My brother likes to drink cola. My sister likes to eat apples and drink water. I like to drink cola. I don't want to be fat. My brother is drinking cola. My sister and I are drinking water.

很好！

I have one older brother and one younger sister. My older brother is fat, my younger sister isn't. My older brother really likes to drink cola. My younger sister really likes to eat apples and drink water. I like drinking cola. I would not like to fat (?). Older brother is drinking cola. My younger sister and I are drinking water.

I have an older brother and a younger sister. My older brother is fat. My younger sister is not fat. My older brother likes to drink Cola. My younger sister likes to eat apple and water. I like to drink coke. I don't want to be fat. My older brother is drinking cola. My younger sister and I are drinking water.

I have 1 older brother and younger sister. My older brother (is) very fat. My younger sister (is) not fat. (My) Older brother very (much) likes to drink cola. (My) Little sister likes eats apple. I like cola, I dont want (to be) fat. Older brother is drinking cola. Me and younger sister is drinking water.

Wǒ yǒu yī ge gēge hé yī ge meìmei. Wǒ de gēge hěn pàng. Wǒ de meìmei bú pàng. Gēge hěn xǐ huān hē kě lè. Mèimei hěn xǐ huān chī píngguǒ hē shuǐ. Wǒ xǐ huān hē kě lè, wǒ bù xiǎng pàng. Gēge zài hē kě lè. Wǒ hé meìmei zài hē shuǐ.

S + zài + doing something

I have an older brother and younger sister. My brother is very fat, my sister is not fat. Brother likes to drink Coke a lot. Sister likes to eat apples and water. I like to drink Coke, I dont want to be fat. My brother is drinking Coke, my sister and I are drinking water. 

I have one older brother and one younger sister.  My older brother is very fat.  My younger sister is not fat.  My older brother really likes drinking Coke.  My younger sister really likes eating apples and drinking water.  I really like drinking Coke.  I don't want to become fat.  My older brother is drinking Coke, my younger sister and I are drinking water.