Is sinus arrhythmia serious disease?

No. Sinus arrhythmia is a normal variation in the heart rate with the breathing cycle. It is not a disease. It is normal.
No. A sinus rhythm indicates that the conduction system of the heart is working normally. A sinus arrhythmia simply means that the count varies so that instead of beating at 60 beats per minute, for example, it is varying between say 50 and 60 beats per minute. If continuous tracings are done, almost everyone has some sinus arrythmia at some point. There is no association with increased morbidity.
No. Sinus arrhythmia refers to the normal variation that occurs in heart rate with respiration. When we inhale our hr increases and when we exhale it slows down. In some people (particularly in children) this normal variation can be very noticeable. Check with your doctor that your heart rate variation is indeed sinus arrhythmia and not a true abnormality of your heart rhythm.