Hi lazyminimoo.
First of all, don't let anyone pressure you into thinking you 'need' another child, even if it is by your mum. It's your body, (although I'm sure you know that!).
My partners grandparents had 6 children, who are obviously all a lot older now, in their 40's. They pretend as though they get on, but in reality I've seen and heard all the different sides of bitchiness from each and very one of them! A big family doesn't necessarily mean a happy one.
My mum was an only child. My nan had previous pregnancy problems before conceiving my mum, and with her birth she lost a lot of blood, so in them days was advised not to have any more children.
My mum had a very happy upbringing and has no bad words to say.
I have one sibling. (Also an older half sibling- haven't seen her for years, she decided to lose contact with us and our dad).
Me and my younger brother had a great childhood. And yes it was like having a friend, like when we used to go and stay at our dad's house at weekends etc. I couldn't imagine an upbringing without him. But that's probably because I can't remember him not being there, as I was only 2 and a half when he was born.
We were very close up until a few years back when obviously you go off and do your own thing (I'm 22 now and he'll be 20 soon). We have both moved out, so don't see each other that often now.
I'm now expecting my first, and although he will have a half sister that doesn't live with us, for now I'm thinking he might be an only child. Of course instantly it makes you feel some sort of 'guilt'.
I admire anyone who has more than one child (and of course people with one child too!) because I could not imagine being pregnant with a toddler running around too! 
Who's to say you're children would even automatically become the best of friends? Or even like eachother?! I've known of siblings to despise eachother unfortunately!
Basically, the choice is yours. Just don't feel pressured!