According to MIL, 9 months is the very earliest babies can walk. 
I could go on... It all comes down to how wonderful and amazing her eldest son is. DH has probably done more really too. But it doesn't register on her scale of worthiness. I've done some pretty cool shit too, but nothing compares to her eldest...
My parents have heard the stories many times from DH. I think they thought it was a little bit of him just exaggerating or playing the victim.
Until the 2nd day I was in hospital with DS and she came to visit. And did nothing but talk about how wonderful her eldest was, and not a single word about how great her second son who had just become a father. My parents were stunned by it.
Its incredibly unhealthy and has caused a rift in the past. We didn't tell the in-laws I was pregnant until we were busted. DH would probably like to go NC again, but he wants to have a relationship with his SIL and Niece (of the Golden Balls). He's not so bothered about his brother. But I don't think he realises how unviable that's going to be. Its been hard enough dealing with it all before.
DS is currently fortunate being the only boy grandchild. What happens with future grandchildren could make life more difficult. And I'm concerned about the walking milestone now. It will effect him I think, as stupid as it sounds. If he is before it, he could end up golden child (which isn't good), or put her nose out of joint (again not good) or if he is after then it becomes that he isn't as good as the golden child and is set to follow DH in the family dynamic (obviously not good). Its ridiculous, but this is the way she thinks. And I can't see I positive scenario, given the way she values things. Its worse than comparing DS to his cousin really.
Personally I don't think achieving milestones early is necessarily a good thing anyway. I think it can be something of a poison chalice. It brings its own challenges to be ahead of the curve, just as much as being behind can be worrying. And generally I'm not sure that walking early has much of a baring on your future career prospects or life opportunities!
A good friend wisely has said that by the time they are three they can all walk, talk and eat and no one cares when they did it. Its so true. Apart from my MIL!
There is a family do, in a couple of weeks. I am now dreading it.