Get another builder round and get him to quote a price while your sister is there and when the shock hits her i think she will help you out a little!
If not dont fit it!
its not one of them things like helping them move when ya dont expect to get paid, you doin a hard job which will take alot of your time...Tho if she can put up with you only coming round 1/2 days a week and living in the house with a bare kitchen for a few weeks then maybe you could do it for free!
you could always lie about the cost of matriels
"yeah these screws cost £10 for two! there very special!"