Op - 2 yrs ago my 2DC learnt that they were beneficiaries of my bachelor cousins will, along with a no of others. It turned out he was quite wealthy and their portions were enough to give them a decent house deposit if invested wisely. They were 17 & 14 at the time, both sensible, but we also spent a lot of time talking to them about nest egg, house deposits etc.
DS was learning to drive at the time and joked about getting an expensive car. He is very much a saver so it was just a joke, but DD gave him a lecture on not wasting his inheritance. She was 14 at the time - less than half your DP's age - but seemingly with more financial sense.
Fast forward to now, DS has had his money a year, hasn't touched a penny except to move monthly from savings into a stocks and shares ISA. He is at uni, enjoying life but careful, and manages quite well on his loan, some top up from us and wages from earlier job.
DD now 16.5, her money just been transferred from solicitors to the control of DH and I. Some in NS&I, rest Initially went into a trust account, but since moved to an ac in her name to get more interest. She could ask me for the passbook, draw it out and blow it. But she wouldn't, far too much sense! And I wouldnt have put the money in her sole name if I thought she would access it.
It sounds like your DP is using you to continuously bail him out, as his sisters are using his Dad. Your fault for letting this continue, but as a PP said, he probably wont change, as he could just turn to his dad for a bail out. His dad is as much a fool as he is.
We have drummed it into our kids that we will continue to finance them through college, uni etc (altho expect DD to get a PT job as her DB did) and let them keep their inheritance for a house deposit. However, if they blow it, they will not be bailed out by us, either generally or for a house deposit.
I would question why your parents still want you to get married. Are they, as a pp said, scared of losing face to friends or losing deposits? If my DD was in your position, I would tell her to run a mile!