Need some honest advice

Sam
Ok so would love honest advice please. 
I'm 28 SO is 39 we have been togther 6.5 years living together 5 years. 
Not married he has two sons from a previous relationship the oldest is 22 this month youngest is just 14. 
The oldest has just had a son himself who is now 4 months. 
Both boys are great oldest just moved out to have his own place with his baby and fiancé youngest just moved back in with us 
( there mother is not the best u fortunately) 
When I found out my SO was going to be a grandad I freaked out lost my mind for a while I called it off ( like 2 months) not just that but that to me was a big where is my life going question and is the grass greener kinda moment. 
We are back togther now and ttc for my first. ( third time round trying after several obstacles mean we had to goback  on BC such as job loss etc ) 
But my question to u all is am I mad to have a child and carry on in this relationship if suddenly his sitstion is not what I hoped eg 
I'm closer to his sons age although mentally defo not. ( always knew that but written down as I get older it seems more strange) 
He is 39 and I'm trying for my first with him 
Any child I have will have a nephew older than it. 
My SO son's fiancé will be a sister in law to a baby ( when married) 
Written down it sounds just so messed up makes a me want to run a mile. 
But forgetting his bagage as to say.  I would not think twice about having a bsby with this man. 
Any comments welcomed x