No house in the world can give you a cuddle to melt your heart.
And no house is going to shout at you in the middle of a pubescent mood swing in a way that really twists in the knife to break your heart as much as possible because it's learned all your buttons - and then have to find space to separate before it fights with their house-siblings who have less say in the matter which adds on that guilt as well.
Really, everything in life has risks - some of the best things are very risky, but we can only weigh them up if we recognize those risks. While it sounds OP like you had a lovely relationship with your sister, it's just as likely they won't. For every story of people regretting not having another, I've heard people discuss how 'if they knew then what they know now' they'd have stopped and even more so for people who say they'd wish they'd had sibling(s), I think there are just as many who'd rather not have dealt with that.
It's hard to say - It's a hypothetical person with all the complexities of the person and relationships bring compared to the complications a new house would bring -- or as a pp suggested, your status quo with the money put into improving where you are.
Maybe it's because I have teenagers, but I'm in the make your life easier camp. This might involve moving - I've done that recently, though the house isn't actually bigger but it has a better layout, with more accessibility features in an area with more available (and it's ours rather than a rental) that even a month in has improved our quality of life in many ways. It might involve other areas of your life to build up with that money instead.
A baby is very much unlikely to do that - knowing people with teenagers and pre-schoolers, that can get complicated. For some that's worth it, but I'd weigh up the worst case scenerios a previous poster (or my above description of teenage doom) if it's mainly a matter of annoying broodiness 