Bridesmaid dress neckline

So this may have been asked and answered at some point but my sister/MOH is in love with a strapless dress for her bridesmaid dress, but my dress is also strapless. I really think the dress is beautiful and flattering on her and I was OK with her and the other girls purchasing it until I started reading stuff saying the bridesmaid dress neckline should not match the wedding dress neckline. Am I overthinking this? My dress is obviously a wedding dress and different from the one she likes. The dress in question does come with optional spaghetti straps which I suppose I could just ask them to use? How much thought into your bridesmsaid dresses are you putting into it when it comes to this? I really do want them to be happy with the dress since they are shelling out the money. Thank you for your input!