child support question????

I am a 35 yr old male whose wife left him for another woman 4 yrs ago. At the first of this ordeal I was left alone to primarily care for my 2 children of 2 and 6 at the time (the oldest not being biologically mine). Now 4 yrs later after much hard work my ex shares the resposiblility to the benifit of the children (with a little coaxing from her family). Financially it is hard as on one income I run the house which is where they go to school just down the road and give them some stability in thier lives.
9 yrs ago a woman I dated got pregnant. At the time it was told to me that it was not mine but another mans. Then 2 yrs later she says it is mine again. As she was on social assistance an inquiry was made as to paternal responsibilities. I offered my self to have a DNA test done as long as the other man in question went also. After learning of this she tells me off and moves to the US for a couple yrs. Recently got married and then separated. Now as she has reapplied to social assistance I recieve a letter in the mail saying I owe child support.
I believe if it was mine....9yrs ago when I offered and didnt try to get on with my life was the time to settle this. Obviously I am not a deadbeat but rather of victim of them. (eg eldest daughter isnt bio mine but she is still my daughter in all respects and recieves all the love and more that my younger one does). If it happened that this child was mine I could no longer hold the house we own and could not provide for them as I do now and thier mother doesnt do much in that respect but I beleive it is essential for them to spend time with her as to form a relationship with her. Any advice or input would be greatly appreciated. I live in New Brunswick and am not sure of the applicable laws here. And responsiblities assumed by her husband. Thanks