OK, so think about this -- your sister/SIL is going to wash the blanket eventually. Maybe not in the first week but soon. Do you want her to do the first machine washing and then the the blanket falls apart?? I'd machine wash it first to make sure it won't fall apart on her!!
Do you still have the yarn label? Does it say it's machine washable? Likely is if it's mostly acrylic.
Mirl56 at Ravelry, too.
Learn from the mistakes of others. You canít live long enough to make them all yourself. Eleanor Roosevelt
Lady Feb Sweater
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