When there is more then one kid I like to give a gift all the kids can share. Harder as they get older, but a board game for all four, or legos, or some art supplies. Something all the kids can play with. Then a smaller individual gift for each kid.
I think she could start mentioning how much Tom loves trains, and Tim is so into robots! I think people are seeing them as one unit, rather then two separate people. Start pointing out their differences. If someone asks what they kids want say "Well Jane is really into One Direction, and John is into skateboarding big time. Tom recently went to a train museum and just fell in to love with all the trains! I think he wants to be conductor, and you know Tim, that kid can't get enough of his robots!", no referring to the "the twins". They're separate.
I don't think it's rude to point out that all four kids have very different tastes and likes and dislikes. I'd also say something like "We know when Tim and Tom were younger it was easy to buy them a gift to share, but now that they're older they're really noticing they have to share everything while their older siblings get individualized gifts, it's causing some hurt feelings".
And a t-shirt?! One twin gets in Monday, Wednesday, and Friday, the other Tuesday, Thursday and Saturday and we rotate Sundays? Was that the plan?
*note: naming twins Tim and Tom is not recommend, typing this post was very difficult*