There's an awful lot of things going on here, goodness me.
Right now, she's a single-parent. She rarely takes action because of work, due to tiredness and partying. She only has time for her during weekend. I told her about it.
First of all- this is awful. I won't go into an opinion-based view here, but clarity on if the Mother has any actual involvement here would be helpful.
Does your Daughter live with you, or in her own place?.
Edit: If the Mother is involved at all with her Daughter, she has a role to play here as well. She needs to be included in making sure that whatever goals, objective and routines are put in place, that she works with rather than against you so that there's clear "teamwork" going on here.
She always goes to sleep at around 3 to 7 in the morning. This has been going on for a month now. She wakes up around 5 to 7 in the afternoon. Before, she barely even slept in the afternoon.
Every time she's gonna wake up "Where's my iPad?"
usual series of tantrums. Scream, cry and throwing the pillows out of the couch.
I always have no choice but to just give it to her.
Later on. She's gonna ask for a chocolate drink. She doesn't like white milk. I don't know. Is it because of the chocolate drink?
When we are at the table, her eyes are at the TV busy. She eats very slow. Like take a bite then wait for 3 minutes before another bite.
Before we begin- to put things bluntly, you're going to have to turn this situation around harder and faster than engine-powered merry-go-round. You're going to have to grit your teeth here. A lot.
Ok, simply- you are effectively playing the role of parent here from the sounds of all the issues you're having.
As a result of this, you need to act like a parent. This is a child, you are an adult- who should be listening to who? You're being put upon by your Daughter from the sounds of it and now put upon by your Granddaughter too, being walked all over. This is unacceptable.
Now, this child is being raised to think that her throwing a tantrum is going to get her whatever she wants, whenever she wants it. You can decide for yourself right now if you're going to allow this to continue, or directly move into action and start acting against it to try and help raise a well-balanced and responsible person or not.
The iPad? You do have a choice. You're the adult in the picture here. All of this is down to personal preference but reallistically it's unhealthy for her to be on it all the time, so begin to perhaps impose a limit.
My own son gets a strict "hour only" policy- it's up to him when, but there's an actual hours timer on it that will stop him using it after an hour, so he can have half an hour in the morning and half later- whatever he likes. If he wants to go on it longer, I have to unlock it. He knows that if it gets broken, it won't be replaced- simple. If he's not behaved that day at school or home, he plain won't get it.
Base it on behaviour, how polite she's being, goals she's achieved throughout the day- you'll have to decide yourself, but the policy of "want gets" has got to stop.
The next thing is sleep- it's scientific fact that children need sleep, even Teenagers need a certain amount of sleep- we all do. Electronics such as TV, Tablets, Monitors- they all emit a particular wavelength of light that has been proven to keep your brain engaged and fooled into thinking that it's still daytime, so letting her use any of these is going to make matters worse.
Her going to sleep between 3-7 in the morning though? How does she even function at school? Does she go to any kind of school? I fail to see how she can learn or operate at all under these circumstances.
There needs to be a blanket ban of all electronics, distractions and otherwise from a set period- say she goes to bed at 8PM, by no later than 10PM then all lights are off, it's time for sleep. That's more than generous.
In terms of Chocolate drinks and diet- this is another case of needing to stop. Everyone needs a balanced diet and this isn't being encouraged here- I'm sure that every kid in existence would love to have what they'd like all the time, but this needs to change. Limit the drink to....say a bottle, carton or however it comes twice a week- once it's run out, it's run out. That way she can have it still, but will have to seek an alternative.
When eating, well it's simple- don't let the TV be on! Many families around the world enjoy Dinner as a family activity, not all staring at a TV. I had a similar problem with my Son so we just turn it off- no distractions, dinner doesn't go cold, everyone's happy. Once it's finished, it can come back on once things are cleared away, etc.
There's a lot of solutions here and not all may work for you- it sounds to me like this girl needs a lot of love, patience but also to be taught that authority and respect are something she needs to understand.
She needs to reduce the time on the iPad, show respect and be polite, reduce the intake of just chocolate drinks, go to bed and actually sleep at a reasonable time and be shown how to act like she should be.
If the developmental cycle she's already exibiting continues, she's going to grow older and older thinking this is normal and will treat other people the same way, which I'm sure everyone can agree is not a good thing.