1) Is your sister-in-law going to use it for free-machine embroidery, or does she want lots of "ready made"stitches? The more fancy the stitches, the more expensive it will be. Frankly, If the free-maching route is followed; you don't need oodles of fancy stitches; just a zig-zag, facility and lots of practice!
2)Does she intend to make a business out of the dressmaking side of things? If she does, it would be an idea to buy a more robust model from one of the well known companies. Remember, it will be in use most of the time.
3) Try out your short list of machines before buying! It is important to get a feel for the machine before you purchase. All brands have different qualities.
This is a matter of matching the machine to her needs and your price range - hope this has been of some use to you :)
oh my goodness, I wish I could be more help, but like you I've had my machine since the year dot and dread the idea of getting a new one!
I have used this firm often though for yarn and the staff are very helpfull when you phone them.