hey hey!!! im happy that ur thinking of donating blood, good for u!!!
well first off, yea, u might not have the same blood type as ur sis, as as for the fainting and feeling weak, i really depends on the person....
they usually recommend that u drink lots of water the night before and eat a meal before u go in for ur donation, so make sure to eat!!! lol!
the first thing they would do is test ur blood to make sure that there's enough blood count so that u wont get anemic... so if u havent ate, or ur on ur period or somthg, ur lil blood sample wont pass and they'll tell u to go home
if ur okay to donate on the other hand, if ur not scared of needles, its really not bad! lol! its quite a big needle and i dont find that it hurts while its in my harm, but then again it could be different for everyone... i just get restless after sitting for so long not doing anything (cuz u cant really move much either, or else ud shift the needle). where i go to donate blood, the nurses are always checking up on me and tell make sure u rest for 5-10 mins after so that ur body can produce more blood before they let u go.. and where i go, they provide snacks like cookies right before i leave.
depending on the person, ur arm may be sore after so they recommend u dont do any heavy lifting, but once again, im usually okay, but its different with everyone!
i hope this helps!!! and please dont be afraid and donate.. blood is greatly needed!! take care and let us kno how it goes!