Yuki, gosh!! your relatives and your MIL are so unreasonable and unconsiderate!!
My niece, 6 mth old bb, just had chix pox 3 weeks ago, after his bro & daddy got it. My mum got to went over to help. I ask my mum not to visit my bb during this period. I know I sound super kia si but my boy just recovered from bad cough, the last thing i want is chix pox for him.
In the 1st place, they should have tell u their son got chix pox, then is up to you whether u r comfortable to let yr son get close to him right!! How can they don't said anything and let their son played with your son!!
Watch out for any blister this period, the incuberation period for the virus is 2 weeks. Once you see any blister, quickly went to PD. My niece was given med to control the outbreak.
Hope your bb will be ok!