I don't think YABU, but this thread has certainly made me think more deeply about these questions.
Re. presents specificially, I've never personally spent significantly more on "blood relatives" (hate that phrase, actually) than on others - eg. I used to buy nice gifts for my late sister's DH, & still do for my dad's partner. And my PIL don't make much distinction between DH & me, as far as I can tell, in terms of money spent. Though perhaps that is because they knew that, for years, I did the present shopping & got them a lot better presents than DH used to!
More generally, I think my PIL would say they treat me as a full family member - but that's not always been positive. My mum died shortly after I met DH, & my dad & I aren't particularly close (DP divorced when I was a teenager). So I think PIL believed I'd just be absorbed into their family, & accept all their ways of doing things.
FIL once said to me that he hoped to be a father figure to me, & I felt quite uncomfortable about it. Just smiled as I didn't want to hurt his feelings, but really I wanted an adult-to-adult relationship. I could see that PIL were both still quite interfering with DH, even though he was nearly 30 at the time, I didn't want them thinking they'd assumed some sort of authority role in my life. Our relationships have evolved but it's taken a long time to feel that I'm treated as an adult, & the same applies to DH.
I've also often felt that beneath the family rhetoric, I'm blamed when we don't do what PIL want. There have been plenty of off-the-cuff comments / digs that very much give that impression - DH sees them too. Some of it seems to stem from the notion PIL have that families must share similar views & have the same tastes. I've even had MIL give me dark looks while saying in a reproachful tone: "DH didn't used to like olives!" or "But nobody in our family has ever eaten rare steak!". 
I do agree that where ILs are often excluded & there's still a lot of focus on the original nuclear family, then it's hardly surprising that the PIL find themselves on the outside when GC come along & their DS/DD & their partner form their own nuclear family. I have friends with very long memories about having been treated as outsiders, & TBH I don't blame them. Nobody wants to be valued purely as a GC production machine, & some PIL do seem to make DILs feel like that.