You need to find other places for your love to live.
You have plenty of it, and it can grow infinitely.
So spend time building friendships, maybe even your relationship with your brother if you can, your friends, your colleagues, volunteering, anything you feel passionate about really, that will bring you into contact either face to face or otherwise, with other people. Especially if you’re helping with or organising something, that’s really the key to it.
You say you have good friends… invest in those friendships too. When your parents become older, perhaps ill, and eventually die, you’ll have a strong and loving network around you. A partner and children may or may not be part of this.
Really good relationships take years and years to build, and many fall by the wayside. There’s got to be more to those relationships that having a coffee or going for a drink. It’s ultimately the shared holidays, shared events you’ve helped at, shared personal difficulties that you’ve helped with, that really build friendships. Introduce your parents to your friends too (maybe in small doses!) so that when they have passed away, you can share their memory with others.
Start now - you have everything to gain, and nothing to lose. Rather than getting stuck on what you can’t do, concentrate on what you can do.
No one will ever love you like your parents, and you’re very fortunate to have that strong bond with them at this stage in your life. But the natural order of things is that they will likely pass away before you, and either or both of them may have years of ill health ahead.
Do what you can now while they are fit and well to shape your life for when they have gone. It can’t do any harm at all!!