Forensic psychology isn't an easy path to riches. Psychology as a degree is incredibly over subscribed. Over 40,000 people graduated with a psychology degree in the UK last year. Then you want to work in forensic psych to get a feel for it. Your best bet is getting a bank role as a Healthcare Assistant in a secure mental health facility. You then have to apply for an assistant psychologist role about £25k a year, also usually only for a fixed time period of a year. Except you're fighting those 40,000 other graduates for maybe 25 available roles at any given time. Often it is nece or desirable for you to have a mastersbfor an AP role. Say you get the AP role without the masters, okay great now to progress any further you need to do your masters.
So now you're looking at about £70k worth of debt just to be earning £25k a year. With a very competitive and stressful infrastructure. Source being: I worked in a medium secure psychiatric hospital for 4 years. Every second person you met was hoping to go into psychology. I also wouldn't suggest MH Nursing either. Every nurse I knew was miserable with their role. I initially started studying for my nursing and had a huge breakdown about the fact I didn't want to be miserable and hate my job like everyone else I met. I then fell into the job I do now which is working with teens in custody.
Now from what I can see a forensic crime scene investigator already earns about £3k more from entry level. It also appears much more straight forward to climb the ranks and progress. Also worth noting that the police and companies they outsource to tend to fund further study. So you could potentially get any post grad training paid for through work. With a specialty area and a good few years experience you could be looking at £50k a year. Which even in the current shambles of an economic climate is still a good salary.
I'd say that your dreams of becoming a forensic scientist are much more realistic to come to fruition and likely to be a lot more lucrative a lot quicker.
As for your sister not supporting you siblings can be mean. She possibly means well but don't let her dampen your dreams. I'd also suggest reading a book called "I want to die, but I want tteokbokki" the starting chapters have some good bits about siblings.