Family offering to be my mortgage. Any pitfalls?

FTB looking at a one bed. Got a really healthy deposit and the amount to borrow to be £90k

I'm stunned that family have offered me that £90k as an interest free loan.

If I pay it back at £500/mo the person would be 75 ( years old by the time it is paid off. We have agreed if they die before it's paid off that would be accounted for in the will.

I'm not sure how it would work if I sell it within the loan period

I'm really stunned and don't know what to think.

Are there any issues with this idea? My head is spinning