When I had my first son my mother inlaw used to take over everything when ever we went to visit her or she came to visit us (ie every day she lived a mitre away next door) as my son got older and more interactive he seemed to preffer time

with Noona than Mummy and I hated it, Not only did she give him lollies galore and feed him till he could bust but she took it into her mind that she could take him whenever she wanted without asking and discipline him how she wanted (smacking). After my second child was born my son wanted nothing to do with me at all and Noona wanted nothing to do with our second child. Now my advice is what I did. Take the intruder and your partner, tell them that you want to have a serious talk, sit them down and lay down the RULES, First tell them that things HAVE to change, You are the mother not the intruder, you have a routine at home and if it is not followed if and when the child stays at the intruders house then there will be no more sleep overs, The child is only small and should eat accordingly nothing fattening and everything in moderation (over feeding is neglect), the discipline is to be done either by the parents or in a way that the parents agree with (eg naughty corner for 1 min each yr they are), and no more favourites if you want time with 1 child you spend the same amount of time with the other. these are basic rules that are  easy to follow and even easier to enforce all you have to do is be firm when telling how it works and be vigilant if some1 slips up make sure they know about it straight away and make sure they know that you are NOT going to put up with it. Remember that you are the MOTHER not them and you know whats best for your child They just think they know and will try their hardest to make you feel like you are not good enough DON'T let them remember that you are doing this for you and your child and you will be fine everything will work out as long as you stick to your guns. And if you have more than 1 child and it seems that 1 isn't getting enough attn (eg they act naughty for attn that they can't get any other way) be sure to pay them some more positive attn older children tend to feel left out when there is a new baby you have to reassure them that you still love them and that they are wanted and loved.