Another Kind of Love

Last week Sara was on the phone and her sister put on her two girls (our nieces). Sara--not on speakerphone--talked to them briefly and then said "I love you" before hanging up.
This might not be thought-provoking to you, but it really was to me.
If I had been on the phone then I would not have ended the conversation that way; in fact, it never would have even occurred to me to end it so. Part of this is because the two nieces are 2 years old and nearly 4 years old, so they were around both before the wedding and even before I really knew Sara. Also, they live 800 miles away from us and I've spent less than ten days in their presence at this point. I don't know them all that well and they don't know me much either. In fact, though they are very warm with Sara they mostly react to me like I'm a hairy-faced unfamiliar adult, which is exactly what I am. The older one is old enough so that she warms up to me after some time, but the younger is definitely not there yet.
And yet this is my family now. After we move to Chicago next year, they will be close family that we see often and regularly for an indefinite amount of time. They will become friends and frequent play-partners with our unborn children.
"Love" is an appropriate word to use, then, if you think about it in this macro sense, but that doesn't change the awkward feeling that I would have when presented with it in the moment. There are different kinds of love, and I'm not just talking romantic vs familial. There is the inner love that we effortlessly produce for other people, and there is also what I'd call the outer love that is part of circumstance that we pull from to give to other people. The love I have for Sara comes from within me that I created (or that her presence or my belief in her wondrousness created in me--deep romantic love can never be fully explained in words), while the love I have for any family member came originally from our shared environment.
My own aunt and uncle tell me that they love me, and nothing seems odd about that. They are family that I've seen consistently throughout my life. Of course we feel a love for each other.
Is it ok that I don't currently feel love for my young nieces? Is it ok to grow into it? I think it should be. Think about it another way: if you are either recently-wedded or have had a partner for a few years, how would you feel if one of your in-laws passed away? If you have not had the time to build up the natural feelings of love for the person, how do you react?
I guess I honestly wonder about this, because my disposition with regards to this stuff is reservedness. I don't quickly come upon love, and when I find it (with one big exception--thanks, wife) I am slow to acknowledge it, especially verbally.
I am a classic midwesterner is this regard. My mom always says she loves me at the end of phone calls, but I rarely return the comment (in fact Sara always pesters me to say it so I do it more often now). I of course love my mother. I have always loved her outwardly and very early on in life I learned to love her inwardly, too. But still I don't always like to offer it up: "I love you."
When my mom talks to me on the phone she does so via the speaker so my dad is almost always there too. At the end of the call, he hardly ever adds his own "I love you." My father of course loves me. But he doesn't always like to offer it up. When he does, I find it more amusing than comforting, because he'll deliver the words in a louder, clearer voice, almost like he's reassuring himself that he says it often enough, changing his cadence as an involuntary way of marking a change in substance, like a writer using CAPITAL LETTERS.

I feel like I need a summary of this post. Everyone loves their parents because they're mom and dad, but not everyone really loves them because they love them; but presumably every husband loves his wife because he really loves her.
A man can let his heart find him a woman, but not a set of parents.
To truly love a person is to not need to use empathy to have emotion for him. I think this is how you know if you love an in-law or a relative.
Is it ok to love a relative only as a relative and not fully emotionally?
Is it normal or acceptable to fully love some relatives and not others?
Does biological immediacy require clean demarcations of emotional attachment, or would it be normal to love a distant relative more than an immediate one?
Is there an initial grace period with new relatives to let love build naturally, and if so roughly how long do you have before wondering if it might not happen?