But wasn't opinion of family members only advisory?
Not really. Evidence suggests that the nuclear family was deeply involved in decision making. But it also depends on what type of decisions we're talking about, e.g. military, legal, customary, etc.
One military example that springs to mind is when the lord went on a Crusade to Jerusalem, acquired land and donated it to the Knights Templar. Since this contradicted the dower right where all possessions, past and future (and future), are to be shared equally between husband and wife, the wife brought this case to their lord and it was ruled in her favour - the husband disposed of her lands which was against the law. Revoking the land itself was then a tedious process.. So that's a more extreme example where there's law prohibiting the lord from acting as he wishes.
When it comes to disposing the inheritance though, it can perhaps help if you imagine that for the better part of the childs life, your inheritance was normally inaccessible to you. When you come of age, you'd get some small part of your inheritance from your parents (while the parent is still alive). Let's imagine then that your father died on a Crusade (which wasn't uncommon for the high middle ages), your mother would then take a half that inheritance. Once your mother dies, you could be well into your 40'ies. If the couple had no children, you'd typically have a widow ruling that land until she dies (looking at some succession lines, you'll notice prolonged female regencies and ruling as widow where the women rules for the majority of the total time that the title existed). Anyhow, a way to allow the children access the inheritance is to allow them to "co-rule" it. Those children might at that point also have their own families and land in a location which wasn't necessarily in conjuction to their parents'. I.e. your grand-children's future are tied to that land and the parent of that child would have reason to influence the decision.
But then it's also the simple fact that the father didn't necessarily have a reason to not hear their children. The entire family depended on the land, and those who stood to inherit had as much stake in it. Like for example where the family as a whole takes the decision that's it's simply better if the seasoned uncle rules, than the male child who is still young. That said, I can't say if there were actual laws to prevent the lord from ignoring his family members, or in which cases that was, as I don't have my books at the moment to find such example. It is possible that this was also the case and that there may well have been areas where the father had a more absolute rule. But from what I remember, I also had the impression that sometimes it was rather the family "pushing" the father around, rather than the opposite. E.g. imagine two brothers and mother forming a "faction" against the father, but I don't have any examples to cite for that, it's just part of how I remember something I once read. Maybe someone else can fill in on that.