Don't ask a family member for a recommendation. Their praise won't have the credibility to impress the admissions staff. But you should also plan ahead. Next, compile a file of letters before you need them, especially once you've started college. Ask for letters right after you've finished a course with a professor who likes your work.
Some colleges can help by maintaining a dossier, or official letter file.
The people writing your recommendations are doing you a favor, so make it easy for them by being polite and organized. Give at least three weeks advance notice. Schedule an appointment to discuss the recommendation fully. If you're concerned that your recommender has forgotten your letter, gracefully remind them by asking if they need more information. Once your letter's been sent, be sure to send a thank-you note to your recommender.