미영 as a whole is not a particular word, but the individual characters have a meaning. For 미, they almost always use 美 (beauty), as you have also mentioned. 영 may be one of 英, 永, or 榮. These are hanja, or Chinese characters mapping to 영. 英 means flower, excellence, talent; 永 eternity, journey; 榮 glory, honor. According to a name database, the above three are the most common, in that order If it's the first one, your sister-in-law's name means "beautiful and talented" for example. 미 and 영 also happen to be the character for America (미국, 美國) and England (영국, 英國).

Well, actually we are naming one's name based on Chinese letter. So, you need to know what Chinese letters she uses, if you really want to know what her name means. But, since you don't know her Chinese letters, I can tell you only this. We usually use "미" for the meaning of beautiful, And "영" for honor.

It depends on the situation since the meaning of letters can vary even though those letters have the same pronunciation. Before those had formed, Korean had been using the specific language named 한자, which was derived from the Middle Chinese. At this point of time, one thing you should know is Chinese characters are representative ideograms. For that reason, Korean characters have the distinctive linguistic traits analogous to Chinese. I'd recommend to ask your friend let you know about her name ;-P

I'll agree with the others and say when it comes to Korean names, you need to know the Hanja for them (the Chinese characters they originate from).