Help! I can't tell my twin nieces apart! 😩😲😜

So my brother got married a few years ago and his wife has twin girls. We live in another city now, so we don't see them as often. I've been studying them when I can to try to notice the minor differences. One is slightly bigger than the other, but as they get older, they begin to look exactly alike.

Any twin tips to help me tell them apart? Thanks!