Linux

best option

Ubuntu is popular and new-user friendly. And xfce is generally lighter on resources. It's a good choice.

What can I do

Almost everything.
Some proprietary apps you've used from windows may not be available, but equivalent ones would be available on linux.
Stuff like browsing the web(provided that you don't open too many tabs, because you have low ram) and watching movies n all is quite good.
What all things fo you intend to do on it? I think it'll be easier to check that the things you want are there.

• Gaming is less stable overall but it's exponentially better now thanks to Valve and the Steam Deck driving support. Like seriously, from a cointoss whether it even launches a couple years ago to 74% Gold or Platinum on the top 1000 games on Steam.
• Programming is easier (you can ask your commandline to install all you need at once instead of having to painstakingly individually install and set up requirements or addons to programming languages), but you don't have access to Visual Studio if you're working on C# or C++.
• Web browsing is identical, watching movies too. I've never had a problem using LibreOffice and OnlyOffice as a replacement for Word and PowerPoint, but I don't use many complicated features in Word or PowerPoint so your mileage may vary.
• Photoshop, Premiere, etc are a pain to get good replacements for, OBS for recording and DaVinci Resolve for editing is a really powerful pair though.
• I vouch for Mint with XFCE too. It was very fast on my laptop and some of the themes now are pretty. It barely uses any RAM. It has a Windows-style start menu and taskbar.

Just be warned that your family members will probably have (usually solvable) issues if they want to do anything beyond web browsing. It's a different operating system after all and it works differently in a lot of ways. Definitely recommend looking up some videos about Mint, XFCE, transitioning from Windows to Linux.

I think Pop OS is a great place to start. I haven't personally used it but Linux Mint is commonly recommended to people who have only used windows before, it tries to have a similar UI experience.

Linux can do most things windows can, except it's free. The best thing about Linux is the depositories, unlike windows you rarely download apps from the Internet, instead you can download them from a repository. In Pop OS it's called Pop shop, it's different depending on which OS you are using.

It has everything you may want to use - LibreOffice, Firefox, Thunderbird ...

Some programs aren't supported for Linux but you can find replacement. For me it has 100% of tools that I need.

When you go from windows to mint - they feel similar (UI), but some things are different (installing programs, settings...)

It is just solid out of the box experience. You don't have to customise it to be usable, complete suite of programs and it is stable.

Some of really light distros for old PCs are missing lot of these things but you may need to explore those options if it doesn't run well.

Think of it as Windows 7 in terms of functionality but with a Windows Vista/XP GUI.

That said, your CPU's performance will increase noticebly on Linux.

There's very little windows can do, which linux can't also. The difference will be in how, and how easily.

Often the answer is just "install the same program, and just use it like normal". Other times, you have to go out of your way to get something running using wine.

For this, bottles is a GUI manager that can make life a lot easier.

Something that uses XFCE is a really good starting point for weak hardware. And mint is a good option for someone new to linux. It is based on ubuntu, and there is plenty of info online on how do things on ubuntu.

It also has good default repos, meaning you'll be able to find and install most software you might need, without having to start fiddling with custom software repos.