My sister-in-law constantly snores whatever her position in sleeping is. Is this a bad sign about her health? She is currently in her early 20's and overweight.

Yes. Sleep apnea can lead to a more serious risk as far as I know. This may also cause discomfort to a person, like difficulty in breathing and poor sleep. Often, this is evident to overweight and obese people. If untreated, it can cause stroke, heart problems, diabetes and make arrhythmia even worse.