Serious question: If you don't go, you still get a degree right?
It is undoubtedly a con, but not one I would be particularly concerned about if I were you. A hundred quid for one of the proudest moments of your life? It's just something you'll have to bear. I know I'd pay a hundred quid for a few prestigious events.
You do still get a degree if you don't go, yes.
No offence OP but I wouldn't take your sister with her newborn baby. LOTS of people are going to want to slap you/her/insert other family member here after a 2-hour ceremony. It's long and boring enough as it is without a crying baby starting up.
I would tell your family your money situation and say that unless they pay for their tickets they're not coming. YOU DON'T HAVE THE MONEY, simple as.
Yes, if you don't go you still get a degree. Most of my friends really enjoyed graduation - but for a big uni they really do it well. Three ceremonies a day for two weeks so each one is quite small and intimate lasting about forty minutes. Only charge is for your gown, and that fee doesn't go to the uni. Guaranteed at least two tickets, but managed to get four, plus a small reception afterwards - all free. Wouldn't have missed it for the world.
There's no way you should be expected to pay for anyone. If you were in a school play, would your parents expect you to pay for their tickets to see you perform? Attending your graduation is optional, and if they want to do it, they can pay for it.
We only had to pay for gown hire; the rest was free or up to you (in the case of photographs etc).