Just wondering, as reading another thread got me thinking. And sorry for long title 
MIL practically ignores DS (he is 6, and from my previous marriage). I have a 3 YO dd with DH. When we are with her, its glaringly obvious that she is uninterested in him and only interested in DD. If she ever babysits, its only ever for DD, it would be unthinkable to ask her to babysit DS :(
For example, last Christmas, MIL spent about £100 on DD, and asked DH to go out and buy DS a present, from her, for "about £10"
and :( ...and its not about the money, I'd rather her spend just £10 on BOTH of them, or even ONE pound, its the glaring discrepancy between them that pisses me off. And the fact she couldn't even be ARSED to choose him a present ffs.
I feel it will only get worse as the DC get older. and become more obvious to them. Have tried to talk to DH about it but he claims he can't see it. and his precious DM can do no wrong in his eyes
I have been with DH since DS was only about 18 months old, so she has known him since he was a baby, which you would think would help form a bond. And DS hates visiting her, and always plays up when we go there, its as if he knows where he is not wanted :(
To contrast, my EX Mil calls both my DC her grandchildren, buys them equal presents and absolutely loves and adores them both. They both call her "Nanna" and I even visit exMIL with just DD sometimes as DD is only in part time nursery.