Thank you, the range of opinions is genuinely so helpful and is in line with me seeing it rationally (DP is wealthier as is their family and they are right to want to protect that) and psychologically (it feels like it’s a deliberate attempt to use their greater financial power to get me to sign away my "rights").
Just want to reiterate it's super odd that everyone seems to think I am a woman and my DP is a man?! Wonder if as another poster said people would be assuming the opposite if I was the one asking for the prenup?
Some context to answer questions:
DP's parents tried to get all their children to sign prenups but all partners (including me) refused so this is a new attempt - they have suggested that without it DP would be cut out of will. There is absolutely no way I would even consider signing it were it not for this pressure.
It is sound financial planning BUT they also have a long history of trying to exert control over various family members using money/legal structures. Normal boundaries with respect to children and grandchildren are not respected. Financial structures are complex. This means sadly even signing a postnup realistically would not mean an end to control and hoops to jump through.
They have already given some £ to DP (probably to try and avoid future IHT) which DP sees as solely theirs but I have read conflicting things about whether or not it counts as a matrimonial asset if savings are jointly held. Our home is in part funded by historical inheritances from DP's family. So I might need to sign away my rights to present as well as future things. (I don’t know how elaborate the post nup structure would be).
I understand my in-laws' rationale of protecting DP's money if we were to split -
DP and my partnership has been uncertain at times which is really key in this and I really want my DP to feel safe and that I do not want to be with them for financial reasons - but also feel they would not be attempting this route unless it would be less advantageous to me than a 'normal' divorce settlement if that most awful thing were to happen.
My concern is that money has always been a big factor for us due to differing earnings, different ambitions, different amounts from families, differing approaches to childcare. So negotiating things with lawyers will bring all that stuff up again (eg how do you put a £ value on childcare/home responsibilities AND earnings?) FWIW, DP's parents' best outcome would I think be that we separate - they have made it very clear that they do not like me - so if the process of necessitating a prenup brings on divorce for them that is probably a bonus as long as it is financially in their favour.
I love DP very much indeed and love our family. I do not want to end the marriage but DP's parents' behaviour is really upsetting me and making me ill - it’s all I can think about and I’m not really going out, exercising, eating properly etc; I cannot see another way to be actually free of them than to either sign it or walk away.