Internet speed question ?

I live in the same apartment complex at my sister. It’s a 3 floor apartment & I am on the top floor with my son while her and her boyfriend live on the bottom floor and our dad in the middle. My sister pays for the Internet and we use her router which is in her apartment downstairs. My son got a new computer last year and to set it up there is a cable that runs from her room to his. I use the Internet to watch Netflix & texts/calls/surfing the net. My father doesn’t have a smart phone and rarely uses it. My sister has complained that her Internet is too slow since me and my son started using it but is that possible? Would too many people using one router affect the speed? Would me buying my own router change the speed?

Thanks!