In terms of personal references, character references, you can refer to her integrity, trustworthiness, steadiness, stableness of home life, honesty, and personableness.
Maybe efficiency is a job skill, but stableness, steadiness, trustworthiness--those are things that a character reference would say.
You have the valuable perspective of many, many years, so that's a good thing to stress--that her personality has always been the same, that her steadiness, reliability, etc., have always been a part of her life.
But the fact that she does you huge favors isn't really the big point--your and your brother's emotions aren't the big deal. And they might make you seem biased, "bribed" or "in debt" even, so don't dwell on them.
Focus on Andy, not on you. As I said, begin your trial sentences with "Andy is" and "Andy does" and "Andy has been."
So: she has keys to your dad's house and is someone you trust implicitly--that's valuable for an employer to know. She is a reliable, cheerful person who is not afraid of hard or mundane work, which you know because of her participation in the sugarbush projects together. She is enjoyable to be around, which you know because you've spent time with her in social situations, in emotional crises, and in work projects.