Do NOT sign
It might indeed be binding
whether SAHP or not, you can't tell what would happen in the future,
how much you might desperately need your share of joint assets
e.g. you could become disabled after accident, illness, birth etc
or current or future DC could require care.
Your marriage could break up after you have been an SAHP for many years and you would need a good buffer before you could be self-supporting again.
If you and your DH have mutually agreed that you became an SAHP,
then it is only fair that he shares assets, to compensate for your loss of current and future potential earnings
Otherwise, you need to return to ft work and your DH must share fully in childcare, admin and housework
However, if PIL are helping with a house, then it is fair for them to ringfence their part of the deposit,
before any split of assets,
but NOT a % of the gain in value, because that is a result of joint work between you and FH.