Ah, EJKiszenia!  I see you encountered one of the forms of wildlife that live in the dense forest of music.  "The wild bore".  Sadly, not particularly rare, but tiresome and it is best to avoid them lest they bite or go after you with their tusks (which will always be made of lovely antique ivory, of course).

The wild bore subsists largely on a diet of what they feel passes for dry wit and the occasional juicy bit of gossip.  One of it's distinguishing behaviors is to always root up some fault with someone else's playing or gear or whatever, and then to justify it as positive feedback if they can perhaps find some morsel that *might* be somewhat true, whether it actually applies to the situation or not. 

Usually a bit of translation will set thing right.

For example "Thumbs too long and flexible" = "Wish mine weren't so short and stiff"

And "Hands more suited to piano" = "Wow, you must have some reach!"

Their protective colouration in the wild may include, but is not limited to, "penguin suits" as already has been mentioned.  It really depends on environment, since some of them will adopt the markings of anything including being more of a "hillbilly" than you are, if it serves their purpose.  It will almost always include what can be passed off as a friendly demeanour, though, since they learn that when they are still sucklings.

When encountered, they should not be teased, fed, or annoyed.  Sadly, hanging is probably too good for them as well.

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Seriously, EJ.. if somebody has to go so far as saying your fingers are "too flexible" to find anything wrong with what you do, then your playing must have really knocked them on their butt!

I've heard plenty of such crap over the years.  Hands too large, fingers too long, hands too heavily muscled, fingers wrong shape, "double jointed", left handed people never do well in music, etc.  One of my favorites from a few years back was a person claiming watching my hands when I played guitar saying they were "freakish" like "hairless spiders" and implying to others I might have some rare disorder.  Apparently such a disorder exists, but one of the symptoms is that it is painful.  I am the only one among my siblings with no sign of arthritis by this age (knock on wood), so I think that unlikely.

But yeah, somebody says something and it sticks in the brain and you can end up in a bit of a funk over it for a few days.  Just ignoring it obviously didn't work, so order a pizza to celebrate and see if you can con a couple friends into coming over for a jam session.     

Oh, and take an award for "Most flexible thumbs"!

...since flexibility of the fingers and thumbs is usually considered a good thing in music, so far as I have every heard!

Isn't that kind of like saying something like "You have just too much independent control over individual finger movement.. It is sure to hold you back!"  LOL

@Mt. Fiddler:  I got much the same thing from my family.  Got told that musical talent just didn't run in the family, and that I should be realistic and just accept that.  That was around 35 yrs of music ago.  LOL

I think that what it comes from is that if there hasn't been anyone in a family or group for a while, it just isn't understood that it takes some time and work and it will sound pretty rough at first.  Especially with the modern world's expectation of instant gratification with entertainment, and the tendency of marketing and media to paint the "stars" as being somehow just naturally talented, people end up with some unrealistic ideas about music or anything else that one has to work at to get good. 

Not saying I don't believe in talent, or that the "big names" don't have some.  But that marketing angle really doesn't give any credit for the hard work and persistence/discipline it took.  It sells even the most talented of artists short, in my opinion.

But anyway, if a family hasn't had musicians or artists in it for a while, then there isn't anyone to remember that it takes work to get to sounding good and it takes some time.  I was told something similar to what your relatives are telling you.  I didn't accept it, and I went ahead with music.  The belief that it "didn't run in the family" or "wasn't in the genes" is not one I chose to pass on to my own children either. 

Keep playing, and hang out with musician friends, folks that understand the musical learning process a bit.