Another simple solution would be for the priest to make the sister a Eucharistic minister. This is no a difficult process, and is done on a fairly regular basis for folks distributing tot he congregation.
Again, though, this is a Catholic answer. Your denominations may vary.
Actually, Skammer and Polycarp, I never went any formal licensing to become a chalicist. I just spoke up when there was a call in my church’s newsletter for more chalicists. I received a bit of half on-the-job training and wound up on the schedule. Then again, I was one of 5 people doing the job and we need two per service.
Skammer, while I confessed (with appropriate shame) to a bit of squeamishness about this, I can’t think of any reason why it wouldn’t be appropriate for the chalicist to come to your sister-in-law, take the host (bread) out of her hand, dip it in the wine, and place it in her mouth. This is a reasonably common practice in my church (about 10-20% of the congregation, at a guess) among people of all ages and levels of mobility. Admittedly we don’t have anyone in a wheelchair there, but I can’t see how there’d be an objection. By all means talk to the person who’s responsible for chalicists. There probably is more than one who wonders what to do in this situation. While you’re at it, maybe you could even look into becoming one yourself, if you like.
By the way, which church do you go to? My former minister may have been one of your priests.
Just a short, half-joking question about what happens if the wine gets spilt on the floor. At my church, you would have a chalicist whose face was redder than the wine!:o More seriously, I would have gotten the priest’s attention and mopped up as much as I could, first using the cloth I used to wipe the lip of the chalice after each person drank from it, then using anything handed to me by the priest. I haven’t done this yet, but it is a recurring (mild) nightmare.
Polycarp, we only use communion wafers, and never had that much left over, fortunately. Mind you, there have been a couple of Sunday mornings when we’ve had more wine leftover than I would have liked considering I had a 15 mile drive home. Then again, I did have 2 other people to share with.
CJ
Thanks CJ. In order to be a chalice bearer at our church, you have to be an acolyte for a while first. While they recently added a bunch of new acolytes, I think there is a waiting list to become a chalice bearer. Besides, if I was at the altar, who would push my sister in law around? 
If you want to see if you know any of our staff, my church has a brand-spanking new website: St. Bartholomew’s Episcopal Church.