Great update! Seems like the warmer weather has really brought things to a new level. Am still amazed at the changes in Linda from when you started posting about your fun with your friend now years ago! Your continued compliments and urging Linda to try things a little more challenging have really bourne fruit and she seems to really enjoy teasing you and others with her exhibition. Hats off to you!
As Marcella seems to be very sheltered and inexperienced, having Linda take the lead [as a couple others have suggested] in getting Marcella to relax and enjoy life may work better. And have Linda lead by example [and you complimenting Linda in front of Marcella telling her how hot / sexy she looks, how much more comfortable she must be, etc.]: short skirts / dresses, leaving buttons on tops undone so more of her cleavage can be seen, braless, sexy sheer knicker and bras, etc. Maybe Marcella could go shopping with Linda and Linda could get Marcella to try on and buy some sexy new clothes. The age difference may actually help with this. Use the same baby steps you did with Linda, two steps forward and one step back was the phrase you used back then. Would guess Marcella may resist at first, but you have to know inside she is wanting to do more and experince the thrills that she sees Linda enjoy - and your reactions [and others!] to how sexy Linda is when she has fun. The summer heat is your friend to get Marcella to shed her inhibitions and excess clothing! And keep helping Linda explore her own adventures!