Is it illegal to picket outside a house?

It could be considered a breach of the peace or harassment.
Usually it is best to consult with your friendly local law enforcement and inform them of said event in advance of it taking place so that it is considered as a peaceful and lawful event/protest.

It is for your protection as much as it is for others involved and as long as it is peaceful, the police will not stop anything as it is a civil matter not a criminal matter.