Not necessarily.. it's more about control I would say.
My DH has a theory, called the sit, stand, lie down theory and when I thought about it and used that analogy against certain relationships I can see that it seems to be true.
stand (top, favourite, lavished with attention and praise, warm)
sit (indifferent)
lie down (bottom, scapegoat, punished by withdrawing attention and praise, cold, ignoring)
If you take your MIL (for instance), the children and their partners (either individually or by association) all have certain 'positions' within the family to her at any one time, it's a very fluid dynamic and she can get the behaviour she wants out of them by manipulating them into certain positions, so for instance if one is pleasing her with the attention and consideration she requires, she will lavish them with praise, attention, favours etc, and openly criticise or gossip about other members of the family to them, making them feel special and better than the others. She will be actively indifferent to the others or purposely doing things as punishment for not trying hard enough to please her.. gushing about the top spot etc, it then prompts the bottom spot to try harder to work their way up the heirarchy, they have to try and win her affection basically, she won't give it freely if they aren't doing what she wants.
The way the dynamic works is someone has to be in each spot, you can't have more than one person in the top or bottom spot, if the top person displeases her, she will withdraw affection and praise and actively give it to someone else in the family, elevating them to top position and relegating the previous top position person to the bottom as punishment for displeasing her, they then do their upmost top get back to the top position because she treats them better there. The new top person tries their upmost to stay there, but ultimately they can't because she is only using that as punishment to make her preferred top spot child jealous.
It only works when everyone plays their roles, but they have been manipulated and trained from children to play these roles and don't realise they are doing it, there is little they can do about it, as if they opt out, at best they will sit in the middle/indifferent position, at worst they will be a scapegoat. You're finding it confusing because you obviously haven't been raised in this dynamic and are taking her indifference/ignoring you as her not liking you and taking it personally, when really she's trying to manipulate you to get the behaviour out of you that she wants, which is to worship the ground she walks on like her son does, and if you don't she's not interested in you and will badmouth you within the family to punish you for not conforming, she's even getting your DH to pass the message back to you so you know about it!
I may be off base, but this dynamic happens quite often in various degrees in families, work places, friend groups etc, more so when there are controlling people like your MIL involved. Most people are happy being in the top spot and unhappy when in the bottom spot, so there's a constantly shifting dynamic with can be quite toxic, but people play their roles because no one wants to be the bottom/outsider/scapegoat.
The only way you can combat it is to genuinely not care about conforming or care about the role that you are given, and don't get drawn into any negative / gossipy behaviour that her or the others can use against you to elevate their own position within the family, just be a nice decent person and concentrate on your own lives outside of her, there is literally nothing you can do to change their dynamic and you cannot make her accept you no matter what you do, as she's only interested in people she can control.