
Kids will “go mental” some times, this is unavoidable and you need to accept that to some extent.
If she’s hurting Grandma, remove her from Grandma and say “I’m not going to let you hurt Grandma” and if she goes mental sit quietly with her till she is calm. In that state she can not be reasoned with no matter how hard you try, so don’t. When she’s calm explain what she did to hurt Grandma. This needs to be done immediately after she hurts Grandma, not later. Kids that age can not link past behavior to future punishment.
Kids also find it easier to understand what to do vs what not not do. Show her how you want her to play instead of telling her what not to do.
As far as time outs go consistency is everything. If you’re consistent it will get easier over time. General do 1 minute for how old they are. 1 year old = q minute of timeout, 7 years old = 7 minutes.

I feel like this issue is a hug problem in America specificly. People are terrified of letting their kids roam free and I believe its one of the factors that’s led to the rise of anxiety in kids. I remembered reading a story where a woman let her son walk to the store alone in her rural town and litterly got arrested for that.
You could get juice or some other healthier alternatives to offer the friend, most sodas really are poison lol. Maybe try offering carbonated flavored water, some times people just want flavor if that makes sense. But, if he declines, just let it go, don’t make the kids feel uncomfortable coming over.
You should probably avoid them as long as you can. But people act like they’re haunted or something lol. They’re tools, like a hammer. Should I give my kids a hammer? If they want a hammer to build a bird house, sure. If they want a hammer to fuck shit up, no lol. Should I leave them alone with that hammer, absolutely not, until I teach them how to use it and they prove they can be responsible.
How old is your brother, different places start Pre-K at different ages? If he’s younger than 3 that’s pretty normal, social play starts at between 3-4 yrs.
If you’re still trying to socialize him try taking him to the park, bring a soccer ball and kids will just randomly join in. Check if the library has any events for his age as well.

I feel like with most things with kids, you just talk to them honestly and openly about things. I feel like, if they’re old enough to ask, they’re probably old enough to hear the answer.
This way they feel more comfortable asking you things. Because even if you lock down their devices air tight, they’re friends parents probably didn’t do the same. You can’t protect your kids from the world but, you can prepare them to face it.
Definitely not their first account either. I’ve been seeing these post way longer than two months.