There has been some good advice on here, so please don't dismiss it.
Whatever happens, seek your own independent legal advice. That is what your DP's family have done, and you need to do the same. I'm not judging them for wanting to protect their assets, but you need to do the same, and to ensure you are not disadvantaged by them protecting their assets.
So, if you get married, then later divorice (statistically quite possible), any of your assets / property / money in the bank is split between you both. If your DH has his protected by a trust then he gets to keep it all. If yours isn't protected by a trust, then he gets a portion of it. Potentially half of it.
I would also be concerned that the house won't feel like 'yours', but if you feel you can live with that, then that's no problem.
The comments about children - say you decide to adopt, it's much more common for the woman to take time off / go part time / take the less stressful job, meaning your earning ability is affected in the short and long term. If you later split up, you're financially worse off.
By taking good legal advice you should be able to ensure you and your DP have considered all these scenarios.
I'm not sure I would believe that you will inherit the house if widowed. They are clearly keen to keep all their assets 'in the family'.
So long as you go in to this with your eyes open, and genuinly understand their reasons, and cover yourself legally as much as he is doing, then I'm sure you'll be fine. Better to put the legal groundwork in now and forget about it, than to ignore it, think everything will be fine, and end up shafted in a few years time.