I personally would never want or expect to have any kind of parental relationship with my (hypothetical future) in-laws - I can't even conceive of having more than a cordial, friendly relationship. I find it very sweet that some people regard their in-laws as being like their own parents/children but until I read this thread it was a concept I'd honestly never heard of or thought of. Unless perhaps someone married very young or was seeking a parent surrogate anyway. No matter how lovely they are, I don't think anyone I met for the first time in my 30s could ever become a parent to me like someone the ones who raised me from birth.
I think the idea that the woman who raised your (hypothetical) SO could ever be a "random" stranger is a bit of hyperbole - if your SO/spouse has a relationship
with their family at all, they are going to be in your life -and if SO has a terrible relationship
with them, they are going to be inside his/her head for you to deal with the aftermath. Good or bad, your ILs are a significant relationship
However, there is lots of middle ground between "truly like family" and merely cordial. I have a very warm and affectionate relationship
with my MIL. She refers to all her five DIL's as the daughters she never had, and really takes time and effort to make us feel welcome, included, and important to her. Of course our relationship
is not truly as intimate as with my own mother, but it is also less complicated and has less baggage. We are very huggy and have good long talks when we visit, we put a lot of thought or $$ into gifts for each other (I have more time, she has more $$). But it would never occur to me to call her three times a week just to "touch base" or tell her what cute thing the baby did, the way I did with my own mom. I have both an individual relationship
with her, as well as a role within the "cohort" of DIL's.
I think the gifting thing is also dependent on the overall family context. In DH's family, there are 10 people in Generation 2. All of the bio-kids are boys, and 3 of the 5 couples have always lived close together. Treating the DIL's significantly differently from the bio-boys would come off like a gender statement, and treating the DIL's (at least the local ones) significantly differently from each other, would look like an intentional slight because they usually are together for gifting occasions. I think if the family were more mixed in gender/family alignment, or more geographically spread out, the relationships
might grow more separately.
FWIW, I also think it is odd to give a significant sum of money to one spouse and not the other. When either FOO gives a cash gift to only one of us, (Birthday or whatever), it is a modest amount of "fun money" (somewhere between a cheap haircut and a decent bottle of wine). It is the norm in our families that spouses pool financial resources, so any significant check would be assumed to be a couple gift.