If you are asking a family member or friend to perform for your wedding, here are some questions to ask yourself to assure your music will have the quality you are looking for.
Do they want to perform for your wedding?
If they are a gifted musician and eager to make your wedding special with their music, you are on the right track to follow up with them. Ask if they are comfortable performing in front of guests. I was present at a wedding where the bride's aunt insisted on having her two adult daughters sing a duet. The daughters made is very clear that they did not want to perform, but their mother insisted. The day of the wedding the daughters refused to use a microphone and they barely whispered their music. Only the guests in the first row could hear them. If you invite someone to sing or provide music, you want them to be comfortable doing so. Do not push someone into performing if they don't want to.
Make sure to hear them perform.
If your only experience hearing a singer is a karaoke performance, ask to hear them perform outside of that setting...unless, of course, you want them to perform karaoke at your wedding.
Can they perform the style of music you want?
Can they perform the songs that you want? If they do not know your songs, do they have the talent to learn the music up to performance level? If they need accompaniment, can the accompanist and performer work together to provide a professional performance? A bride that hired me asked me to accompany her singer who happened to be a co-worker of hers. When we met to practice, the singer was very fine on two of the songs, but the third song completely threw her. It was a country song and she didn't sing country. After trying and trying, she refused to perform that song. I recommended to the bride a talented vocalist that I knew would be able to perform the song beautifully. Luckily, the bride allowed my vocalist to perform the country song. The day of the wedding, the co-worker performed her two songs beautifully and my vocalist aced the country song, twang and all! Everyone was happy!
Just last week I met to practice with a vocalist who is sister of the bride. I can see why the bride asked her to sing. She has beautiful voice and has performed for other weddings. Since the accompaniment she was used to hearing were band versions, I offered to record my piano accompaniment part for her to practice with on her own so she can feel comfortable with the piano part.
Hiring professional musicians will assure that your wedding music will performed by talented experienced musicians.