A tip I read from Diane Gaudynski is to have at least 3 of the blue water soluble markers at your disposal for marking a quilt. Alternate them throughout the marking process. This way they last longer overall. I always have at least 3 working blue water solubles and if one starts acting like it won't recharge for me I go out and buy a couple more.
Do know that simply spraying water on them is not getting rid of the ink and it can re-appear. The only thing that gets it totally out is a thorough soaking either in your bathtub or in the washer. Not sure why you wouldn't want to wash the quilt before gifting it. If you do it in cold water and air dry it flat or block it, the quilt won't look washed (as in getting the crinkled look) at all but it will be clean.
I recall reading here when the discussion came up about washing prior to gifting that someone had gifted a quilt and the recipient thought she had ruined it after the first time she washed it. She was afraid to tell the person who made it so hid it away. I can't remember how the quilter finally found out and assured her that was normal. Also it is a good idea to wash to ensure you won't have any seams popping or a bleeder fabric in the quilt so you can fix those issues before gifting.
I think a lot of non quilters could think that they ruined a quilt after the first wash because it does look different.
The quilt should be washed before gifting so your sister won't think it is ruined after the first time she washes it and it looks so drastically different. Especially if you are using a batting that will shrink like cotton, an 80/20 blend, bamboo or wool.