How long were they with the ram and did you see them both served?
It's possible they came into season on the same day, or nearly - sisters often do - but it's equally possible they have different cycles. If so, her due date could be anything up to 17 days after her sibling's. And that's assuming they both cycled and held first cycle; if the ram was with them more than 17 days then it's possible this one didn't cycle or didn't hold the first time around, so then it could be up to 33 days later...
As to her size, she may be carrying twins or even three, so could be quite a bit larger than her sister without it being an indication of a problem.
When you say her milk is in, what do you mean? Have you squirted milk from the teats? If so, I'd think an explore may be indicated. If you just mean she has a bag - well, some of them can have quite a bag for weeks before they lamb...
Are you keeping this sister on her own now? She will be miserable if so. I appreciate you want her handy in case of problems, but she will be depressed if she can't at least see and talk to other sheep. Personally I'd run her with her sister and check for swollen vulva, springing udder two or more times a day - maybe bring her in overnight if you're really worried but let her be with her sister during the day.