Does sinus tachycardia increase your probability of a sudden cardiac arrest?

No: However, there is a reason for sinus tachycardia which may lead to further cardiac difficulties. Get checked for possible reasons.

Depends: It does increase the risk in certain situations - in infection, after severe injury, or after a heart attack (myocardial infarction). You can have it from exercise or ever stress alone without much increase in risk... Unless you have heart disease.