hoping someone can answer this.

i have a few 2x10 joists that have horizontal cracks.

each joist has only one crack.

they all appear to originate from where the nails
went in to secure the joist to the joist hanger.

these joists also have bridging in the center span.

my question is, if these cracks are stable over
years and have not grown in length. is it safe to
presume they were from the original construction,
and not from being stressed ? (ie. heavy loads on
top).

the other question is, if the crack originates
from the end, where it's resting on the hanger,
wouldn't a load on top, tend to compress the crack
together ? (since the hanger is providing some
resistance from the bottom of the joist).

and thirdly, what does "sistering" the joist
actually do ? it would seem to be futile, since
the sistered joist, would need to be in a hanger
too to support the weight wouldn't it ? (i don't
see how just lag bolting another 2x8, 2x10 next
to an existing joist is going to help that joist
bear any loads that are vertical - since the
sister joist isn't resting on anything - just
lag bolts - and the forces are still transmitted
to the existing joist hanger of the existing
joist).

thanks in advance for any responses.

basically i'm wondering if i should worry about
those cracks or not - they are stable, and have
not changed in size. the joists in question are
also sorrounded by doubled up 2x10 joists, 16" away.