D.C.
tell MIL that you'd appreciate her giving love and affection to the baby so he will know he is loved because that is probably why the 3 year old is so sweet.
Other than that just avoid it and be extra attentive to the baby when she is there tending to the 3 year old. In every father children are loved and treated differently and it really is not ideal but not as big of an issue as it might seem.