Cuddly, you can get an income from a range of investments, not just property. You just need to be realistic about the income that the pot of money will generate.
Tbh, it's unrealistic to just expect half the monthly rent. I've been a LL for a number of years and the cost of maintenance, professional fees, insurance, boiler packing in etc mounts up so it's not reasonable to build up a pot of a few thousand before you start taking money out of the rent (that's even with no mortgage).
If you arrange for your brother to pay you off or sell the house etc at least you won't feel like he's not doing sth in your best interest and he won't feel like you're ungrateful. It'll be much better on your relationship.
In terms of investing the money to generate income, you could get help from an independent financial adviser. The money could gradually go into stocks and shares ISAs - there's never any income tax on those investments but you can only pay in 20k a year.
Another option could be a private pension - even if you earn very little (or maybe even not earn at all?), you could pay in £2880 a year and the government would top it up by 25%. The lot would be invested until you retire. You could then draw the money tax free (up to the threshold).
So anyway, there are many ways to invest money (property currently not a great option anyway). In your shoes, I wouldn't mix business with family but get the cash out and then help from an independent financial adviser to invest wisely.