Sorry to hear about the biting. We had a vampire piggie 'Vimto' that literally used to turn round to snap and bite you... It took a around good 7 months but she is no longer a biter, she was just very timid and scared. She started off by just freezing on lap but then gained the confidence to bite, she has drew blood countless times.
With both your piggies
@shaz_dubz and
@Ayla you could do with trying to find out what is causing the nipping, do they want the loo? Are they just feeling insecure? Maybe try cuddling them in a cosy? With Vimt's we started again giving lap time and cuddles for 1 minute then popping her home and built up slowly with food treats, it took us around 4 months to get to a 5 minute lap time, honestly she was savage and I'd almost gave up hope of taming her for lap time. She will now happily lie down on my lap

So just keep at it.
I hope your finger feels better soon, it really does hurt....