my son did the same thing with my mom- and she took care of him while i worked since he was 3 months. around 6 months old, same thing, hated my mom, loved my dad. she didn't take it personally, she just respected his space and he eventually warmed up to her. now he LOVES going over there- both my parents are spanish as well and they speak spanish to my son, so maybe the language change scared him a little bit- or maybe it was just one of those things. if mother in law has anxiety about making him cry, he may pick up on it too.