I'm going to come at this from an HR perspective as well as personal opinion..
This woman was hired with a specific schedule PROMISED. That is none of your business. Unless your boss is an idiot, he probably pays her in the same regard, benefits included (or perhaps not?)
You might recal the readings a few weeks back, about the land owner that sent workers to his field first thing in the morning, again later at 9am, 12pm, and 3pm... All of them earned a days wage. Except the ones that started in the morning who were PAID what they negotiated somehow felt those that came in at the end of the day did not deserve their same pay... or that they should get more... Do you remember that? Was it the business of the field workers how the land owner spent his money?
NOW, if this woman is to help you with experiments, you need to make it clear (sounds like you are a bit sr. to her???) how long they will last. If it will go beyond 3:30, you need to inform your boss, you need a different assistant, as his arrangement with her imposes on your ability to finish your job. Otherwise, you need to inform him that you don't have adequate staff to meet his needs. Inform you boss how her schedule IMPACTS your performance. It's all about what you can get done in the alotted time and what you need to get it done. He either accomodates that or doesn't. That's HIS choice.
Are you paid for the work you do? Fairly? Or going rate within this economy? If so... you're job is as you expect it and CHOOSE to do? As you have negotiated? The fact you choose to put work before your husband is your choice. It's no one's fault. There are other jobs out there. They are perhaps not in your field or at your level. But I suspect you are smart enough to get a 9-5 job. If that time frame is more important than the type of work you do. It's all about what's important, and priorities.
What I would recommend with regard to her constant commentary is NOT to talk to her about the subjects she brings up... How hard she works. (I think it's odd to assume that what she does comes easy to her.)
Rather I would say something like... Look, I don't get off work until this project is done. If that means 10 o'clock tonight then that's it. What I need from you is to keep the chatter down so that I can leave as soon as possible and get to my family. You see, I work very hard too.
I personally don't put up with coddling people. I wouldn't do it for you, or her. I would tell you. Look, this is the job you signed up for. You knew what you were getting into. Tell me how this woman impacts YOUR job performance, and we'll address that. To this other woman, I'd say, look, I can appreciate that your lifestyle only allows you to work certain hours. But you're driving me to the brink of insanity talking about the long hours when I pull an addition 7 over you daily. I feel like your standing infront of a starving person with a sandwhich complaining about how bad it tastes. I'm not discounting how hard you work, but I'm a bit fed up having to stroke your ego about it when I'm killing myself over here.
NOW... even further. You wouldn't be the first person who ends up working in an awful environment. Where the fact that your boss is suprisingly considered intelligent enough to tie his own shoes. We've all been there. You're going to have to make some tough decisions. You might, for example, end up like I did at one point. Working for an INSANE, cruel woman who goes out of her way to give you one set of directions, and then scream at you for not doing something totally different. Admitting she doesn't put ANYTHING in writing 'cause she doesn't want to be held to it. And the boss letting this go on with everyone that comes through. This woman angry with me at one point because I refused to lift heavy boxes and squat for hours filing their contents while I was mid miscarriage. A messy affair to say the least. AND SHE KNEW IT. This was not even part of my job description. Just something she thought of on the fly when I told her I was well enough to sit at my desk and do my regular job. Seriously, I was not down with that. When I finally became PG, but was classified a bit high risk, AND DH & I were quite used to living on BOTH our incomes... I literally walked out the door. Due to one of her crazy rants. I didn't even consult DH. I just left, wrote my 2 weeks notice (in a fashion that scared them to death), collected severance and was on my marry way.
We lost half our income, and I never looked back! You have choices in this matter. Don't allow that craziness of others hold you hostage. Change fields if you must. Science teachers are hard to come by for example. Realize this woman doesn't really work into your life equation...She's a co-worker. Nothing else. The only relationship you're bound to, is the one with your husband...