What should I do if a member of my family partially abandons prayer, and will my sin be like theirs, despite my keeping up with prayer and my shyness in enjoining good upon them?

You do not bear the sin of one who abandons prayer. However, you must overcome your shyness and denounce the evil, clarifying the impermissibility of abandoning prayer and delaying it beyond its prescribed time, and that this is among the gravest of major sins. You can do this through indirect means, such as letting him hear a lecture on the virtues of prayer, bringing him a booklet, or speaking with someone older than him to advise him.