Firstly, & seriously , I'm sorry about whatever tragedy you had to experience, & of course that it caused you to endure a period of homelessness.
Secondly, & less seriously ...
Maybe if you stop viewing your close relations as pets it might help.
It seems that you feel entitled to know private & financial business of the pets you disapprove of or don't like, & feel you have to basically pet-sit the ones you do:
I met DH and DH stayed a lot with me and my DM in a really small cramped cottage and she never batted an eye, happily fed him etc he was quite miserable when I first met him and her attitude to life, the way she was and I really helped him. it was no big deal at all to use to house him, take him to dinner, feed him at home
You make him sound like you found him in a rescue kennel!
He moved into pils large house quite quickly as he was from a different country working in banking and was saving money, I dont know if he paid rent at all.
As it should be.
Your BiL & PiL's accommodation arrangements & whether BiL paid rent is none of your business.
Then sil married him to keep him in the UK
Fucksake. Again with the petsitting mindset.
Maybe she married him because she ... you know ... WANTED to?
and pils gave her a deposit to buy a flat. They just gave it to her with no strings or contracts. They did the same for Dh my Bf at the time
So your PiL's were startlingly generous & scrupulously fair?
but he had to have a contract drawn up, I was not allowed to contribute and it had to be done legally I guess to protect him from me.
I guess nothing of the sort.
I guess that they might have liked to do exactly the same for SiL - but there was no point, as she was marrying her guy, so even legally documenting any protection for her was no longer viable, as upon marriage it became shared money anyway.
But it all came out in the wash, as DH married you, so you now enjoy exactly the same advantage & privilege as SiL & BiL on this non-issue.
PiL would have known that before giving away the money. So they've hardly mistreated you on this, have they?
Its sad for me that we dont have a great relatinship with them, they are not good with the dc and sometimes i want to scream...you backed the wrong horses!
It is. It's sad, & it must have felt horrible to be treated differently & felt disapproved of. But it's been years, & it's time for you to drop this rope because the only person you are hurting by gripping on to it so hard is yourself.
PiLs haven't backed the wrong horse.
The horse is still obsessing over the wrong rider.
Chuck that rider off your back OP.
Your MiL sneers at your degree? So what! maybe she doesn't have one, & feels insecure. Maybe she's just a bitch. Maybe she's got you all wrong & doesn't know how to say sorry. Who cares? You don't like PiLs, they favour DH's sister - let them get on with it.
You've made your own family. Drop the rope! - stop trying to make PiLs give you the attention & favour you wished for from them. It's not happening, & obsessing over it isn't going to change them.
I agree it's unfair. 
I'm just puzzled as to why you are still chasing a rainbow, when you could be putting all that energy into people who actually give a shit about you.
Stop licking this old, old, wound. It won't heal until you leave it alone.