Hi K.,
My twins were born when my oldest was 18 months old. At 18 months old she was potty trained and a good baby.
Once the twins came home from the hospital, the change began! My 18 month old started taking off her underwear and peeing and pooping in the corner, she started taking off her sister's diapers when they were sleeping, she would push them really hard when they were in their swing, anything and everything she could do to make them cry she would.
I now know she was just wanting attention. I imagine for a baby who was getting all the attention, it was really hard to share with 2 new babies. She was no longer the baby and it was really hard for me to split my time because the twins required so much attention.
Now that they are 8,7,&7 I can look back and see what I should have done.
My advice to you is first, to make the 2 year old part of the babies life...."being big brother". Let him help with changing, bath time, feeding, play time etc. Make him feel important.
Second, make time everyday for just him. He's probably insecure and needs some extra one on one from Mom. Pick a time and a project the two of you do everyday. Let him know that is your special time together.
Third, my daughter would react to positive reinforcement a lot better. If I tried to put her in time out or if I yelled at her she seemed to thrive on it. She would do it more. If I ignored her bad behavior(within reason), but really made a big deal about the good things she did...she would get really happy and try to do more good then bad.
Good Luck...it gets better.
C.