Please, I would like any advice possible. :] ( A lot of reading, sorry.)

[Joxn]

So to start, I've had my own boa for a few years now. He's always been tame, and never bitten me once.
Recently I went and adopted/rescued a larger boa.
She's around 7ft long, I'm going to guess that she's been near neglected recently though. She was pretty thin for her size when I got her, and the previous owners of her were not feeding her the correct size of rats needed.
I'm not 100% sure if this is the reason or not; but recently the larger boa has completed two full shed cycles before my 4-5 ft boa has even completed one. My guess of this is because I've been feeding her larger rats than she's been given in the past.
Alright, I'm somewhat rambling to try and give as best description of the snake as possible.

Here's my main concern though. I've never been bitten by this snake (Yet.) Although she has bitten my sister twice now. I feel that she may have been bitten for a couple reasons. She usually takes the snake out to show her off to friends ( Who are usually loud, and I'm not sure how she holds this snake since she's so used to holding the smaller one. ) Both the times she has been bitten has been while she takes her out around friends though, so I'm guessing the amount of people make my new snake nervous?

The other day I tried to go pick my snake up a couple days after I feel my sister added even more stress to the snake. She and her friends had music playing quite often. And by music, I mean LOUD base. So I kept telling them it would aggravate the snake further.

Here's where my questions start now. If a snake was slightly neglected somewhat in the past, is there anyway to help her become more friendly again?
Because she has now hissed at me twice ( The first one happened the day her eyes became blue a few hours after. And the second happened either the same day she finished her latest shed, because she shed during the night.) I'm not sure if this aggressive nature will pass or not?

Here is the biggest concern of mine though. The tank she came in is a very large 120 gallon semi-custom made aquarium with a very tall stand. I'm not the tallest person, I'm 5"7 or 5"8. So it's quite a a task to remove this rather large snake since I have to basically lean over the side with a chair, and try to pick her up from above. ( Which I feel may not be the best way? Since with my other snake I can just pick him up from the front, and slide him out. )
I feel with all of these stressful factors, will it become hopeless? Or is there always a chance to help a defensive snake feel safe again?

(I've mentioned her having bitten my sister at her own fault on two occasions. But, I've held her a few times now with no problems whatsoever.) Just after the recent hissing, and the most recent bite to my sister. I'm concerned she no longer feels safe in her new home.

Additional info; Her take in generally 85 degrees, and has a fluctuating humidity level that I generally keep from 60%-80%