Just that really. I have noticed that my own mum and MIL called their MILs 'Mum' but I never have, and neither does my brother in law's wife. I kinda feel like I would like to; I have been with my husband for 14 years, married for 8, and she is very lovely, supportive and does a lot for us; I love her to bits. But I would not want to offend my own mum, who also does a lot for us etc. and don't want MIL to think I'm odd after all this time! Maybe I'm overthinking it......