J.H.
Please do not use punitive measures like spankings or timeouts. This can really backfire. I had a well meaning educator who told me I just had to be tough with my strong willed boy on potty training. I tried to force the issue. He ended up constipated on medicine for over a year. He was horrible in school because he didn't feel well.
The constipation is a self fulfilling cycle. When he pooped it hurt so he withheld which made it hurt worse. When this happens kids have an aversion to going at all because they know it will hurt. It is self preservation. He eventually potty trained on his own schedule at about 4. I tried everything to potty train him - stickers charts, videos, candy, etc. He was so strong willed he had to do it when he wanted to do it. Incredibly bright kids can also be very strong willed.
Also, making her responsible for cleaning herself is good. Don't make potty training a negative experience. You have a very young baby. This can also cause some family issues as well. Your daughter has been the "baby" for over 3 years. My son and daughter were closer in age than your children. However, he definitely saw some advantages to being the baby. Pointing out what things that big kids can do that babies can't is good too. As well as reaffirming to her nicely that big kids use the potty to poop and pee. Isn't being a big girl great!
The upside was that his sister who was 19 months younger trained really fast because she memorized all the potty training videos. I did not believe that my son would ever become potty trained even though everyone said he would. Luckily his birthday is in the fall so the cut off for getting him into school was later as well.
Good luck