Brian said he is infertile.I think it's fantastic she can do this again for them , I'd imagine this time the donor egg has been fertilised by Brian's sperms.maybe she feels this is the best of both worlds she has an extra special bond with these babies but not all the responsibility and knows those children will always be closer to her than maybe the other aunties.
I know a few female couples who have had a male relative be their sperms donor , maybe a brother of the girl who wasn't going to be the pregnant one , so then they both feel the baby has both families genetically involved .
I wish them both well and aoife a healthy pregnancy