C.C.
There is probably a little jealousy going on. He's at that age where he wants to be a big boy, but he sees all the attention his sister is getting. He may be nervous about preschool - then again - baby sister gets to stay home and have all your attention.
I doubt that it's anything medical, but I guess it never hurts to have it checked out. Is he only doing this during the day at home? And he's just peeing - not pooping? If so, and he stays dry at night, I would be inclined to believe it's an attention thing.
Threatening diapers is probably not the best thing to do. His sister is in diapers and that's just more of the 'attention' thing.
You might try a reward system: you can use a small calendar - hang it in his room. Every night before bedtime, if he has stayed dry all day, he gets to put a 'star' on that day. At the end of the week (or whatever day you choose) - when he has 7 stars, he gets a special treat (it could be a trip to get an ice cream cone, or a special toy he's wanted or puzzle ... anything). At that age, it would take very little to be a 'special treat' ... This would only work if you are not in the habit of getting him treats (toys/ice cream, etc) whenever you are out shopping or something.
This would also help him learn his days (letters) and numbers, so it would be a good learning tool for him. Mostly, kids LOVE to do stickers. It puts HIM in control of the situatuion.
If he has an accident one day, it starts all over. He has to get 7 stars in a row (or 6 or 5) before he gets a reward.
Has he already been enrolled in preschool? Has he visited the campus? It might be better not to talk about it much. If it is a place you can take him to let him see it and become familiar with it, that would be great. Is he used to your leaving him (at Sunday School, Mother's Day Out, etc)? If not, you might want to start doing that before preschool. It would help him get used to your leaving him and reaffirm that you will ALWAYS come back for him. And when he is not with you, he will be safe and have fun and do new and different things.
But, he may be doing this to try to control you in addition to getting more attention.
Good luck.
Good luck.