Fifths Disease?

Courtney
Some background: My husband and I haven't been out together without the baby since she was born 3 months ago. We planned a date about a month ago but had to cancel because she was sick. We planned to go out to dinner on NYE and his parents are watching her. I'm SO excited. 
My SIL just told me they think their 3 month old has Fifths disease, which is highly contagious. She's been planning on spending NYE at my husband's parents' with her baby and 4 year old. She's still planning on it even though the baby may be sick. She says she's not contagious anymore, but she hasn't taken her to a doctor so we don't even know if she has anything and if she does, how do we know it's still not contagious?
My mama brain is telling me not to risk it and just cancel our date and stay home with our girl. My selfish, human brain is considering still having them babysit because, for the love of God, I JUST want to be alone with my husband, for the sake of our relationship.
What are your thoughts? I'm trying to trust myself more as a mom which would require not going out, but, also, we don't even know if she's sick. I wish my SIL would just keep her sick baby at home, but she's planning on still going, so there's not much I can do there.
Help, please!