"As regards capital gains, your sister is not making any gain, so doubt you will have to pay anything on that."
Capital Gains is not on the basis of actual gain but, in this situation, on the basis of a (potential) gain made on the open market.
In reality, if the sister is on the Deeds ( and that is not clear) any transfer from her to OP or Bride to be is going to be riddled with potential tax issues e.g. stamp duty, CAT and CGT.
If OP can get around all of them ( and depending on the figures they may not be huge) then it is a question of asking EBS to join in one Deed transferring interest of sister to brother and/or Bride to be , releasing sister from mortgage and having bride take on mortgage obligations of sister. EBS may decide not to do that.
Better take legal advice from someone who can garner all the relevant facts.
mf