I'm pretty new to quilting, so maybe there's a better idea, but maybe a "true love's knot" where the strips have that woven appearance? Use one color for each twin and explain that it is a symbol of how you know her twins will always be held in her heart?
Perhaps do this as one central block and fill in with something a bit simpler so it goes together faster? Say if you did a large central block (like the size of 4 blocks together) to be that, then did maybe a 4-patch in the same colors to sort of frame it?
Or instead of the knot, there's one that is the same concept but an actual "woven" heart shape (Olfa has a pattern up of that right now, though there's shows 4 hearts coming together; I would just do one, but oversized, and then frame it with 4-patch blocks in the same color, with sashing between the sections of blocks -- ie, large woven heart in center; sashing around that; "border" of 4-patch blocks in the 2 colors; sashing/border; done. Make sense?
whatever you do, I know she'll be touched.