>>322359405
They can and it's entirely plausible to use a sister as a surrogate so the child has the DNA of both dads, it's just not very common.
But, like I said, families are extremely overrated.
We as a species are very quickly moving on from having the merit of life be based on how many kids you can pop out.
We're not in the 19th century anymore, we're learning more and more to live within our means and the parameters our advanced technology provides.
In another 100 years, the family as we know it will cease to be and all the current efforts in enforcing these biological urges will be for naught.