Silly Rabbis. The penalty for that is in Deuteronomy:
Deuteronomy 25
[5] If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of an husband's brother unto her.
[6] And it shall be, that the firstborn which she beareth shall succeed in the name of his brother which is dead, that his name be not put out of Israel.
[7] And if the man like not to take his brother's wife, then let his brother's wife go up to the gate unto the elders, and say, My husband's brother refuseth to raise up unto his brother a name in Israel, he will not perform the duty of my husband's brother.
[8] Then the elders of his city shall call him, and speak unto him: and if he stand to it, and say, I like not to take her;
[9] Then shall his brother's wife come unto him in the presence of the elders, and loose his shoe from off his foot, and spit in his face, and shall answer and say, So shall it be done unto that man that will not build up his brother's house.
[10] And his name shall be called in Israel, The house of him that hath his shoe loosed.
Obviously, Onan was not put to death because of refusing the obligation to give his brother an heir. He was put to death for the plain meaning of the scripture in question, and all of Christianity has understood it as such for 1930 years (at which time the separated brethren caved on this moral theology issue).
Silly Rabbis indeed.
If you read the actual passage where the act takes place, you will see why God killed Onan.
http://www.christianpatriot.com/onan.htm
Your family practice levirate marriage?