Although he should consider the concerns of his family members because they care about him and his life, the ultimate decision of whom Rob Kardashian dates is his alone. This is especially true for a family that has a propensity for drama and attention seeking. It's not even clear if they have legitimate reasons for disapproving of Rob and Blac Chyna.
If he's happy with Blac Chyna then that's the only reason he needs. You can't choose who your family is but you can choose who you choose to be with when it comes to friendship or love so that's why he shouldn't be too concerned with how his family feels about his relationship. His family has no right to control him just because they have a problem with Black Chyna, that's their own problem(s) that they can worry about and address that ain't his. At the end of the day they'll still be his family but he'd lose out on someone that makes him happy.
Rob Kardashian should not stay away from Blac Chyna, despite pressure from his family. Although family is important, and cannot be the overriding factor in deciding whether to pursue a relationship. It is sometimes difficult for outsiders to understand a relationship. Furthermore, if he broke up with her, he would resent his family.
Nobody outside their family should think about that. We don't really know what really went behind the scenes there, you can hear stories, but you will never know what really happened. The Kardashians should handle it themselves, and I am sure they will, not that anyone outside should care for it.