You've got some good advise. I have the same issues and did my will. I opted for my sister, then brother because of the age thing BUT because they live further away, I also said that if my parents were willing (they are close) they could be guardians the remainder of the school year and live at my house if they wished during the transition to the house being sold (which because of life insurance would not be immediately necessary) I also made it a progressive list 1) sister or if she is unable or unwilling then 2) my brother, then 3) My parents if they are willing 4) if none of the above is available, I want my family and friends to meet and decide what they believe is best and a guardian appointed to give advise and a judge to decide, putting in a paragraph about my feelings about the important considerations and the people I have in mind. In there, I did say that if my certain named people felt they could do it (they could now, but may have health issues in another 10 years), that I would prefer them. So what if its wordy- its the document that will speak for you, and your words will be considered.
Also, it turns out that my brother is getting divorced. This is not a problem for me, since the guardian I listed is HIM, not he AND his wife. I would want my daughters to be with him, not be part of a possible future custody battle between my brother and his wife.
Keep in mind the parents of each of you will continue to be grandparents, and the dynamics of that relationship will therefore not need to shift if you leave this earth before they are 18. Reflect on how you can create as few "losses" for them as possible. The sibling is probably the best. As much as I trust some of my church friends I've known my whole life, who live in the right place, being with family is probably more important than anything.
GOOD FOR YOU for thinking of this so carefully. yone