Of course you may never feel the same thing for your son in law that you do for your daughter. But in my opinion, Christmas gifts are not really the best forum to express that. There is no reason to draw attention to biology at Christmas. There are times where it is more important, of course. For example my parents in law have 3 children, my husband being one of them. The three children are listed by name to inherit their property in equal thirds. This is 100% expected and completely normal. Two of the children are married, and neither I nor my sister in law expect to be personally named in their will and the money to be split in fifths! But Christmas presents? That's a time to be inclusive, not exclusive. That's the time to say "you are a part of the family and I will treat you as such".
Also, when it comes to relationships and feeling close to someone, I think sometimes people neglect the fact that gift giving, if done well, can actually cultivate the very relationship that you don't yet have but want to have. If you make the effort to really find out what someone likes and find a gift that suits them, in general that can only help solidify and build a relationship. Of course if you don't desire a close relationship that is something else entirely, but if you want one, I think equal gift giving and making an effort to find out what this person would enjoy would go a long way.
Obviously opinions will differ.