As has already been pointed out, the types of relationships we have has nothing to do with how long the relationships last. Relationships grow close and distant, change in form, and begin and end when they do, and that is all due to how the people involved work at them or not, grow together or apart, and generally find their lives changing.
I think it short-sighted to enter into any relationship without understanding that it may turn out in any of a myriad of ways, none of which are easily sussed out prior to starting. Cultivate them and nurture them as best you can, with an eye to the long-term possibilities to have the greatest chance of success.
To use a carpenter's rule of thumb--measure twice and cut once. Consider every step involved with merging families with a careful and patient eye, then do it again before advancing. Step by step, consider what the effects are of what you're doing and adjust as necessary.