question? Is he crying for your attention or because his sister is upset?
My niece (25m) does this with her sister (12m) for a while now and usually its almost an empathy thing. My sister found explaining that 'sister' is ok she just cried because of X really helped. But I don't know if your is the same. She also gets anxious/ upset when my DS cries, but not as much.
DS 10/09 DS 2/17/11 Blessing #3 sometime 2/13