I don't think it's a shit idea OP and I think 'lady bountiful' is what people say to put others off being kind and generous. I think it's a lovely idea and I understand why you want to.
First thing I would wonder about, I suppose, is whether you or your sister would want to move there with your families if you are currently renting? But if you both have other plans that mean you can make better use of the capital, and if you're both in agreement, there's no reason at all why you couldn't explore something like your idea.
As PP have said there are schemes where you can offer the house to your local authority for a set period of time to be used as council housing - they take care of everything and pass an agreed rent on to you, I think.
Or if you want to sell, it is true that there would be a lot of interest and some potential complications. One way around managing this would be to look to sell to a housing association who can then use it as 'affordable housing' - similar to council housing, or with stipulations it be used for key workers for example - or offer it as part of a shared ownership scheme to help a family get on the housing ladder, and who will be used to administering all of the contracts that go with these schemes. I think it may be possible for you to set up to sell it on a shared ownership basis yourself but I also think it would be legally a bit more complex to arrange.
Rather than the caveat about re-selling in a certain amount of time, I wonder if you could cap the re-sale price in some way so that it always remained 'affordable'. I have no idea how to do this but it's another thing that housing associations are able to do so it is possible!
Another option you might consider is to offer to take part in the scheme to host Ukranian refugees. It wouldn't be renting but you could make arrangements to set it up so that you and your sister weren't out of pocket either, just not making a profit on 'rent'. Whoever is co-ordinating the hosting in your local area - likely to be a church or mosque leader? - would be able to talk to you about options.
I hope you can find an option that you like OP.