Nobody really notices other people's behaviour towards there family because bad behaviour usually tends to be hidden from the general public.
Very few people are nasty to apsolutly everybody the people who they are nasty to tend to be those involved in actual relationships with them so whilst they may be perfectly nice and polite to the gas man they won't be to someone they have a relationship with who they arnt very keen on,we seam to care more about what strangers think of us than we do other people,that's probably why you havent seen any of this unless you are on the reciving end.
I know I prefer to actively choose who I form relationships with and one would think most people are much the same about things like that.
Obviously your born and you grow up so you have no choice who your own parents and siblings are yet you are expected to form a good relationship with someone who if they went involved with your adult child or the family of your partner you may not choose to have any associating with,I'm pretty sure that's why it can be so fraught for the people in those relationships.
I think that problems can start when a commitment is formed because it kinda like shoving in your face that now you have to form a relationship that's often more than just a transient one.
Or when children come along because when your own parent in there excitement oversteps the mark you can quite easily tell them to back the fuck off knowing that at some stage down the line they will probably forgive you due to all your history and involvement but you can't really get away with doing that to someone else's family.
I can remember once when my mother was totally overstepping yelling "what exactly were you doing at 2:43 on the 17/08/93." Her response "I was with you whilst you gave birth" to which I replayed " well fucking remember you were watching me,not doing it yourself and back the fuck off" at her.
She stormed off but got over it because she had to in our relationship I was the only person she had to get past as my DH would have said "your dd you deal with it I won't get involved" where as if she had fell out with dh in her mind she still had someone with a whole life of involvement with her to work on to get her way ( even tho I would have said exactly the same thing as dh she was not to know that) I think its much the same to your partners and there parents they rely on the whole life involvement.
Its very easy to over step the mark with a baby who you are related to because baby's are mostly the loveliest things and are so exciting and I think lots of female relations who haven't had a baby in the family for awhile can often get hung up on the lets pretend the baby is mine thing even if they don't let on whilst the mothers of those baby's are experancing a perfectly normal biological reaction to having there own baby ( that being generally protective about everything to do with that baby) but its easy to forget that when our own baby's are adults so if your not careful you can do unfixable damage.
And no matter how much you dress it up no matter how much you don't want it to be true when it comes to baby's that are either not yours legally or ones you didn't give birth to,they are not yours and its always going to be down to you to act accordingly and not attempt to behave like they are yours.if you don't or can't then you are going to have huge problems.
Oh this isant exclusively female relations that may do stuff like that my ex fil has recently had my child's name tattood on him ( first tattoo so its not as tho he already has his own dc's names done) its weird.