Bummer. But the reality is, she will train in her own time. Look at us, here we are all grown up and not peeing in our pants. Well, at least when we're not sneezing or jumping up and down. Man, pushing out those bowling ball babies really takes its toll! :) But I digress.
My first two were trained through the night by age 3. The third one still had accidents at age 5. But was accident free before kindergarten. When you do some reading the experts say developmentally 3 is really the age kids are really ready to potty train. However, you'll hear the amazing stories of moms that have their kids trained before 18 months. More power to them; I say.
I would not do the time out for wet pants, it sets a bad precedent for when there is truly an accident. She may be peeing in her pants on purpose just to see what happens and how it feels. Or, maybe the other diaper wearer in the house gets more attention than she does at potty/changing time and she wants some of that?
If you think you want to test this theory, try having her help you change the baby's diaper. Talk to her about how proud you are of her that you don't have to do this for her anymore. Then, the next time she does go on the potty give her some lovin'. You know the same kind you give the baby on the changing table: a blow on the tummy, some big smiles, some toe tickling, etc. Positive verbal and physical encouragement goes a very long way with kids that age. Then as she gets in the habit and she'll stop coming to you for acknowledgement and will quick get back to whatever fun she was having before she was distracted by the potty. But don't be surprised if she does regress to needing attention again, just go with it and give her a hug.
Some kids like charts and stars. You keep the stars in the beginning and give them to her when she comes and tells you the good news. Give her a different kind of sticker and give her control over the stash of these stickers for the poops since she obviously has control over this function and it's location. Tell her she can have the star stash too when she stops peeing in her pants. She may like doing her own pooping stickers so much she'll want those stars too. Then the chart is hers, whether its accurate or not isn't important as long as she owns it and her potty training too. In fact you can include her in the creation of the chart so she really does own it.
On that note, remember it is her potty time. She has to control it. Using the timer in the beginning is great, but now that she's demonstrated the ability to control herself, you controlling it with a timer would probably be a step in the wrong direction. Use a natural consequence of peeing in pants to give it the negative connotation you want. So, if she's watching TV when she wets, the time it takes to clean herself (yes, she can do this too with a baby wipe or flushable wipe.) and change her clothes takes time away from the TV. Prolong that time away from the TV or activity by explaining to her that TV time is over now and don't let her go back to what she was enjoying at the time of the accident. Then when you give her a gentle reminder the next time she's in the middle of something fun to take a quick break point out to her that she can come back to the fun activity, but if she chooses to wet her pants the fun is over. You'd have to be strong on this one, especially if you are out of the house. Be prepared to drop what you're doing (play ground, friends house, out to eat, anything) and end her fun. She'll remember that for a very long time.
Speaking of regression, don't be surprised if there is some when the second one starts training. Just go with it, but encourage her to set an example for her sibling and go back to that hugs and kisses for a job well done support system. It'll be good for her and a good example for trainer #2.
Remember the big picture, she won't be peeing in her pants as a grown-up and it's up to her to take control of her own body. You can't do it for her. This is probably the single largest factor in our frustration with this or any part of parenting. But fortunately for us, this is also training for us. We can't control our kids, they are not truly ours to control or own. They are His and our job is to love them and do our best with His guidance.
Take care and know that Diaper Liberation Day will come to your house too; I promise.
K.