Wet Pants

Just wondering if anyone has experienced or is experiencing a once toilet trained toddler is now not. My boy (nearly 4)was toilet trained about 2 1/2 and was doing really well. The changed happened when his sister was born and we were having wet pants on a regular basis. I was pretty sure it was the adjustment of the new arrival and just beared with it for a while. Well she is almost a year old and we are still having wet pants up to 5 times a day. I have tried stickers charts, awards, removing loved toys,ignoring it but nothing has worked. Number two's is no issue they always make the toilet but it is just number 1 and he seems to not mind wearing wet pants. He is still in a nappy at night and I am eagar to get rid of nappies altogether but if he can't manage it during the day i am sure he won't at night. I am just beginning to wonder whether is a deeper issue that I need to address, but just wanted to check whether anyone else has experienced this.