RV, the problem is that some people with or without reason, consider their own mother to be like Christ - the saviour and Messiah and they automatically juxtapose the MIL to be the anti-Christ and she-Devil.
A MIL is not, merely because she is the MIL, a DIL’s enemy. A MIL may not give her life blood, sweat and tears for her DIL like a mother will - but all MILs should not be presumed to be likely to push their DIL in front on an oncoming train.