What are the chances of second baby having heart defect, if the first baby had a defect?

Usually 3-5% Heart defects occur in 0.8% of all babies. They occur in 3-5% of all first degree relatives of people with congenital heart defects. Some of this depends on your first baby's defect. There are some defects that are associated with specific genetic abnormalities, in which case the risk would be higher.
It depends. It depends on why the birth defect occured in the first place - some causes are environmental, like if mom was drinking during early pregnancy, and others are genetic, including a chromosome rearrangement, one major mutations of a specific gene, or many minor mutations in several genes. The risk could be as low as 2-3% or maybe as high as 50%. A clinical geneticist can get you an answer.