"but I'm a bit concerned she may keep it up"
You don't say how old they are but yes this needs nipping in the bud.
She has not yet created a trust with you, and the more you push this by trying to stroke her, the more she will resist.
Since you have a more friendly one I would feed them, then stay with them after the feed. Take a bale of straw in if you need a seat, and just sit down and do nothing, this will encourage them in to you. When either approach, totally ignore - if you try to stroke they are likely to go back. Just sit in there for 5-10 minutes AFTER they have eaten, and let them get curious about you. Hopefully they will come up and get bolder. They may start to explore the straw and you. If either attempts to bite, just tap their noses, otherwise totally ignore them. Then just leave.
Do this for 2 nights minimum - totally ignore. Day 3 if (and only if) you can stroke without them seeing (we wait until they come in close and then just touch their backs, using nails to give them a gentle scratch) then do so, but don't push it. May take a week to get to this stage with the vary one.
By being passive, you take away any threat to the piglet, and they tame much easier. Keep persuing and you will just make the problem worse.
Patience, slowly slowly catchy piglet !!
Good luck