What happens if you don't carry out someone's last wishes

[Meruem]

So I'm talking with regards to their funeral. My mum will have virtually no one at hers, only me and my sister and the only reason I'll go is for my sister (long story but numerous reasons why). She also didn't pay for a funeral plan despite having the funds to, so me and Dsis will have to foot the bill. I'm thinking direct cremation would be better (she does want to be cremated). And that's not me being totally heartless, I plan on having direct cremation myself when the time comes. It just seems a bit much to pay for a full blown church funeral and all the extras she wants for only 2 attendees! The time hasn't come yet but she was mentioning it again recently so it got me thinking. Apparently it's all written in her will. So, is it some kind of legally binding thing? But then I can't see what penalties could be imposed if we didn't follow it. But I have no idea on this.

It’s her wish but no, it’s not legally binding. Likewise, you & your sister do not have to foot the bill; normally that would come out of her estate - if she has any assets, savings etc.
If there is no money, you can apply to her local council for help to pay, or they will arrange a direct cremation ( no mourners) and you can claim the ashes afterwards. Or not - they will generally deal with those too if required.
I am sorry you have clearly got some unhappy history there, but do not lose sleep worrying about her wanting a funeral you can’t provide.

Speak to your sister, if it’s just the 2 of you then direct cremation and a graveside service with the vicar if you want to bury them, if not just you and your sister can do something together when you scatter them. If she’s not paid for it and expecting you to and she expects more people, then no.