Less of an AIBU and more a 'this is why you don't trust gossip' 
DH and I are staying at PIL for a couple of days. Cousin's wedding on the weekend and we do visit them every couple of months anyway. Pre-DS (first grandchild), everything was fine. Since having DS six months ago, MIL has been a bit annoying, e.g. constantly wanting to hold him, not giving him back when he cries, wandering off,... I wouldn't trust her for 5 seconds to look after DS and not start to offer him chocolate and other crap, because "we did this and you were fine". You get the picture. DH is good about boundaries and all in all, it is mostly an annoying habit we try to train her out off (with minimal success).
Quick background, MIL is still friends with many of the mum's from DH's school days. It is very much keeping up with the Joneses and we constantly hear about what DH's friends from school are up to before they ever tell him themselves. There is one friend she is particularly close to, A. A's son got married a few years ago and every since I haven't heard a good word about his wife. DH's local friends went on about how difficult she is and that she constantly tries to show off. Granted, one example was 'I said I want to lose weight and she offered for me to come along to the gym with her', which doesn't sound too bad to me. A. does not seem to be the biggest fan either but is busy showing off her son's and DIL's achievements and probably would eat a broom before she admits that.
Anyway, they had a DD a couple of months ago (3?), A's first grand-child, too. You can imagine the one upping between MIL and A!
Yesterday, MIL popped over to theirs for tea while A's son and DIL were visiting. I have only heard the story as told to DH by MIL but apparently A was holding her sleeping grand-daughter while MIL was visiting. MIL used the opprtunity to tell A's DIL that she never get's to hold her grand-baby, which was apparently met with "well, new mothers have plenty of different reasons why they want to keep their babies close, I am sure you'll get plenty of cuddles when he is older". I am not sure how much DH exaggerated the story but apparently MIL than kicked off asking whether she implied I didn't trust her with the baby and when asked why she would think this, just huffed and puffed until she finally spit out something along the lines of "why else would they not let me hold DS - they clearly don't trust me, so cruel etc etc".
Here is why I want to invite A's DIL over: After MIL's long rant, she apparently replied with "if that is the only reason you can think of for a mother to keep her baby close, then you clearly don't remember these early stages. Going by your reaction, I would want to keep an eye out, too" - "But you leave A alone with her!" - "Because she listens to us and we trust her".
I have set DH on the case to talk to A's son and get the details but he has not heard back yet but I am a little more giddy than I should be and don't want MIL to notice, hence why you have to listen to this Will she be my boundary queen? Is that why people hate her? Stay tuned!
(long-time lurker, previous account owner, back because, well, there is a baby now)