Not read the whole thread yet, but how's about this:
Let's say your sisters name is XXX. Go to your mum and say. "Hey Mum, you know your will is a 50/50 split yeah? Well I had this great idea! You know XXX has been living in your other house all these years and would probably want to continue living there when you pop your clogs, why not leave her that house and adjust the 50/50 split accordingly? So I did a cursory check and the other house is worth 300k, why not leave here that house and add half of that to my half to keep it 50/50? In fact, why don't you give her the house now to keep it simple! What? What is this piece of paper? Oh its just a silly little legally binding document to record this discussion, nothing more. Anyway, this is great because it was my idea!"
Edit: Alternatively, if you genuinely do not care about the money, go to your sister and tell her you know about the house gift and you are totally cool about it. Say, you don't care that your mother gave it to her but you are worried that you mum might get upset if you bring it up with her directly. So please can you XXX please let her know that I know, and that I am cool about it and I would be very happy if she could talk about it to me.
Basically you are giving you mother an off ramp, whist simultaneously getting into your sister's good books to the tune of 300 grand, and her no doubt scheming **** of a husband, but you can't have everything. Maybe once this is all out in the open you mum will spontaneously go along with option 1 above, only this time it would be *her* idea.
You have to really make it sound genuine you are fine with it with your sister though, otherwise she will twig and they probably start to scheme even more. So really lay it on thick that you really don't care about the money, and you are happy she has a permanent home etc. Basically you want your sister to go to your mum and start saying "Oh mum, what a relief, your son Potatolord, came to me and we had this really heartfelt talk and he is 100% ok with the house. But he is worried you'll get upset, please let him know the next time he does your shopping for you, and we can all relax"