@StressedTbh No matter the absolute factual accuracy of some aspects of MiL’s story, she is deeply hurt still & carries the trauma of her upbringing - you made brief mention - then rape & forced adoption.
Apologies in advance for the essay…
Roughly, MiL is late 40s & now has the joy of being a grandmother to your two DC, yet that proximity to a young mum having her first child may have brought forth comparison to her own first born at a similar age to you. Maybe a form of reactive depression or past PTSD that a GP &/or psychologist may recognise & provide sustainable support to help see her through resolving this. Not just meds, if at all, but specific counselling/therapy.
You don’t mention her drinking patterns other than she has booze in the house & gets drunk with you - just with you to release or with others ordinarily? It is an aspect of alcohol misuse or problem drinking that is is a release or cover for depression & trauma.
In a way I can see what her comment about you being the ‘daughter she never had’ being more that you are the ‘young woman she could not be.’
Your SO - bless him, but he needs to knock the only child / no siblings whinnying on the head. For anyone vaguely understanding of most women’s lives, we all know that miscarriage is frequent & although relatively rare so is stillbirth. Not every conception leads to a live birth.
Your MiL brushing this off does not help either your SO or her. She is not going to give him the talk about women in general he needs, so this may fall to you to speak to him firmly but kindly about the experience of womenkind. There is often a reason why only children remain so, & discretion is the name of the game. He needs to have this made clear for everyday use.
However, that said, why does he persist with this lament? Because most only children can accept their situation with simple gentle knock back or explanation. Does he sense that she has a secret or is this his way of dealing with her emotional distance. From what you mentioned, she did had a very difficult time as a child, is there some form of emotional distance between her & SO that is a legacy of that neglect & abuse? Is it her way to brush things off, a way of dealing with difficult feelings or subjects? Her release seems to be watching the ‘reunion’’ programmes & overwhelming you with her burden when drunk? I wonder if your SO has a lurking hunch that based on what he knows of his mother’s upbringing, the ‘reunion’ programmes & ‘daughter I never had’ that something or someone happened before he was born?
The situation is complex & sad.
Not of your making or responsibility.
This is already too much for you .
MiL needs professional assessment & help.
MiL needs to decide to seek help from GP but making that first step to talk to someone may be far too difficult for her to do… on her own.
Is it possible for you to carve out a task with firm beginning to end boundaries to get MiL in to see her GP (or the GP in the surgery who specialises in MH)? You may need to be by her side as she rings to make the appt, possibly sit in waiting room with her & at a push go into the appointment to give her moral support.
It may seem contrary for me to suggest putting your head in the lions mouth after saying that this is not your problem to resolve, but the situation is not going to improve left alone. My proposal is that you are an agent of change to get MiL to people who are trained, experienced & can help her - they go home at the end of the day.
Now that is a huge reach, & many a daughter might not do that for their own mum. You will be entirely reasonable to immediately knock this idea on the head.
What are the benefits of you being in the room with the GP?
It sounds like MiL may have clinical depression (& it is for the GP to discover & determine the diagnosis) brought on by birth of first grandchild - that’s where you are relevant . It’s important to state to the GP that she leans on you for support but you cannot manage this - you are early 20s with two very small children, she’s not your mum - & the trauma of her life experiences are too much for you to support. It is really important for GP to know so that there is a need for professional support.
Is there a problem drinking issue as well - GP will likely ask more if she (or at a push you) mention that her grief & sorrow floods out when she has had a drink.
Make clear to the GP that you now have to step back as this is now affecting you, your relationship & your family.
As the consultation is barely eight or so minutes, not a huge amount can be discussed, so likely MiL will have another appointment to discuss more & go into the relevant detail. Your task is now completed, you have got her to the gateway for professional help, so time for you to step back.
Now this is a very large step to take for you to be that involved. It really is. But I have put it up for something for you to think about. Fingers crossed MiL would not need that much support from you & she can make & get to a GP appointment under her own steam.
Of course there is the real & likely possibility that she refutes the need to see the GP, says that there are no problems etc. You will have planted the seeds about seeking help & support, that is enough. The situation will continue & you really do need to step back to look after yourself.