Need advice on a personal situation - I don't know what to do.

I've got a sister who is older than me and has special needs, she currently living with our brother, both parents have passed away. My brother also has a wife and a daughter.

I live on my own and I currently rent a three bedroom house, my brother recently made contact asking about my sister moving back in at some point as they want to have baby number 2, they've told me I would have another 4 or 5 years before she does.

I feel a bit pressured and I feel like I've got no choice except to allow her to live with me but there's stuff I want to do. I'm going to university this September and I don't fully know what I want to do after graduation. I am keen on joining the British army but can't decide on regulars or reserves.

I also don't want to stay in the same county, my sister does, I want to move about a bit before I start settling down. Not only that I was considering downsizing to a one or two bedroom flat so I could use a spare bedroom as an office.

I personally just don't know what to do anymore. I just feel forced, even though they said I have another 5 years before having baby number 2. My brother claims they have no room but I they have a master bedroom. My thought straight away is the master bedroom is massive, the parents could put their daughter in the master bedroom if they had another child and move into their daughters current bedroom which is a double bedroom.

I feel lost and pressured, my heart feels like I need to but I just want to be able to finally live my life. Any advice?