Things have turned out ok.
The older sister wasnt calling for any reason other than genuinely wanting my company, i got to vent to her though and she said my feelings were understandable and not unfair and that she understood, and also told me not to worry that things were going to be fine and that my partner wasnt leaving me over this.
I went to her place which was nice, she kept me distracted from my own thoughts (i am my worst enemy) and helped me get my head together a bit.
I got quite anxious when my partner arrived because i just didnt know what was going to happen. But things are ok, we are both just very stressed and so both had a meltdown, but we are ok and it doesnt appear to be her going to leave me. She also said she loved me! When she said it my heart almost burst with relief.
Her younger sister is a completely different person. She has been nice to me and i havent felt uncomfortable or ostracized or anything. My partner is still sleeping in the other room to support her sister, but im ok about it. Knowing that im not losing her makes it not seem as hurtful.
I got kisses and hugs when her sister was in the bathroom and such and things seem to have worked out ok.
I am very relieved. I think this weekend wont be the hell i was afraid of.