I cannot personally tell you about black person's hair.
I do have a friend who is white and married to a black man. Her son's hair beautiful. He's 7 now and she kept it long for years as they were stationed in Guam and he looked like the natives, lol. The only problem she had was it drying out and she put lotion on it.
I have seen on Oprah's that experts with black's hair stated that even though the black community says not to cut the hair as your in-laws have stated, it needs to be trimmed so it will grow and also hair will get split ends, all hair and it needs to be tended to like any other. So do trim it now and again.
As far as texture changing, I can say that generally that baby hair with white children will usually straighten out and lose curl, but this is hard to say with black hair as it will be dominant in the genes.
I would say do some research online with some pros and that a trim now and then would be needed, for growth and health.
K. B
mom to 5 including triplets