This is very good advice. But if you are totally overwhelmed, consider these modifications:
Will: if you were hit by the proverbial bus tomorrow, who is in the picture? husband? second marriage? step family? If married, you must get a will, in case husband re-marries after you are gone then leaves everything to his new wife (it has been known to happen).
If it is just you and you are either fully divorced or never married, then if you died without a will I believe everything would simply be split between the three children - but this would not protect your relative's money. Given that you can't leave the cash to the relative, you would be looking at a trust or similar - which is way to complicated to fret over just now. When you do get around to sorting a will, beware of any arrangement that means paying solicitors a 'management' fee for years to come for the relative's pot.
Living will: what if you were in a coma? what if you get dementia? what are your wishes? - again, no rush (fingers crossed) but have it in your mind to get to this, and have conversations with your children about the possibilities.
Then sorting the chaos: get everything onto the computer - scan paper documents and type up scribbled notes onto Word documents. Take attachments off emails to save in folders. Log on to portals and download files to save in folders. Ensure everything is in labelled folders on the computer, ready for when you want to look at it properly.
Make sure your children will know the passwords to everything if you die - this is part of preparing a will / living will.
Relative: put "their" money aside in two new separate accounts - one for quick access to pay bills that might crop up in two to three months, and one for longer term with a month or three month's notice of withdrawals, to get better interest. Don't make it complicated. Yes you could consider various investment options, but you don't have the mental head space to worry about it just now.
Pension: you need to look after this. It is a priority. You can't put the kids university before your pension.
Business: If you are generally overwhelmed by your business paperwork, get a book-keeper. This is much cheaper than an accountant if they are just putting all the numbers together into accounting software for you and tidying the invoices. You need to have tax returns done and know your earnings for the next step - which is Uni costs.
Uni: the system should tell you how much you are expected to contribute based on your earnings - get the kids to do all the necessary research and paperwork, it will be good practice for them.
Then take a deep breath and get some counselling to help sort out your emotions before worrying any further about money.
Then the rental flat: just sell it if you don't want to be a landlord. You don't need the hassle. Don't fret about whether it is a 'good' time to sell, just do it.
Or, decide you will give it the headspace it needs, that you will be a proper landlord and commit to it. Make a decision.
Then finally get an IFA to help you with tax and investments, if you think the amount of money you own is worth paying their fees. If the fees are too high, maybe just do your own research in your own time, without feeling guilty about whether you are squeezing every penny to work to the max. There are more important things in life.