Yes, I generally do give a gift if someone is having an engagement party. But TBH I hate giving money, it just doesn't sit right with me iykwim? To me money just isn't a gift. I much prefer to buy something personal/sentimental for people that they will keep and remember. I still remember who gave us what for our engagement and wedding (we got engaged 8 years ago and married 6 years ago) and cherish those things. But the money we were given is long gone and to be honest I can't even remember who gave us money or how much. So to me it's not a true gift (I'm not sure if I'm wording this right...).

Given that it is family, I think it would be nice to give them something special. I've often given the 'Love' book from the MILK series (a coffee table book with a beatutiful collection of photos representing love) as an engagement gift. But even something like a nice photo frame for them to put a picture from their engagement party in is nice iykwim?

If it was me, I would try and give something a little bit sentimental because they are family. Or give nothing at all. Given that she has said no gifts I don't think they would be offended if you didn't give anything either.