It is not right in any way to proceed for khula at a time when you have two children and that too attaining their puberty. You should read relevant topics from the books like Bahishti Zewar and other authentic books and it should also be read out at home. In-shaAllah, it shall guide the spouses to settle their family affairs in good way and to live in good conduct. If you involve two or three relatives who are influential and you make your bahnoi and sister ready to live together then it shall be far better. Otherwise, in talaq usually the matter of children becomes very critical and also the life of husband and wife become miserable. It is also not easy to do second marriage.
Allah knows Best!
Darul Uloom Deoband