Engagement...or Entrapment?

I learned today my younger brother-in-law proposed to his girlfriend.  I honestly think the family is quite shocked about it - although they won't readily admit it.

Granted, it isn't my place to judge, as my own romance moved quickly.  However, his girlfriend (whom we've only met a few times) has been adamant about talking about marriage, his finances, and other topic that you don't normally broach around your loved ones family members when you've just met them.

I guess since he is younger than me, I look at him as a little brother.  I feel a little sour about the whole thing because I feel protective.  I feel like it is going to be a big mistake, but I have to hold my tongue.

Items which have made me uneasy:
-He hopped at the first girl who gave him any attention
-His mom, dad, and brother all stated when they first met her that they felt he could "do better"... which they no longer admit to saying.  The comments now are, "...Well...as long as he's happy. . . "
-She rushed into talking about marriage/buying a house together.  I get that couples have these discussions, but WHO talks about it in FRONT of their loved ones family during the 2nd/3rd time of meeting them?
-They've been dating 6-8ish months
-She makes cheesy comments to butter my mother-in-law up.  Example:  She had only met my husband and I for the 2nd time.  She positively GUSHED about how WONDERFULLY my mother-in-law raised her two sons.  It was a little overkill.
-She's a nice girl as far as personality goes, but I feel like "physically" they don't match up.  That feel so callus to say, but... if you saw them on the street you would honestly question, "What does that guy see in her?  He could get someone so much hotter".  Which, isn't the important thing in life, and looks fade, but it still bugs me.
-She openly talks about how lucky she is that her boyfriend makes so much money.  She brags about how much more money he makes than she does, and that it is nice to be pampered.  It felt awkward.  My husband makes more than double what his little brother makes, but I don't openly blabber about it.  Nor did I comment on his finances IN FRONT OF HIS PARENTS while we were dating.  It just makes you seem like a gold-digger.  Honestly, when my husband gets a raise I STILL feel embarrassed to ask him how much more he is making - and we're married!
-She pushed him to move in with her.  She's now pushing to buy a house together and openly talks about that in front of family too.
-She hasn't met any of his family except his mom, dad, brother, and myself.  They have yet to be around grandparents, aunts, uncles, cousins, etc.

I just worry he's rushing it so fast because she's pushing him.  I worry that he was so lonely for companionship that he jumped into a relationship with the first girl after college who provided it.  I worry he sees his brother (my husband) so happy in a marriage that he wants to be in one too.  *Sigh*  Why in the world am I so worried about it?  He isn't my blood relative... he isn't my brother.  I mean, he's 24 years old for God's sake. . .

I guess it is because I truly view him as a little brother, and I'm protective of him.  I don't want anybody to hurt him.  He's a sweet kid, and he deserves to be happy.  I just hope his decision to become engaged was the right one...for his sake.

I'll force myself to bite my lip (except to my husband) and happily wish them congratulations tomorrow when I see them.  I'll try to make it as genuine as I can...but something in my gut just says that this is a big big mistake.