Charles I and Henrietta Maria had five daughters - of whom, only the eldest (Mary) and the youngest (Henriette Anne a.k.a. Minette) left surviving issue. The other three - Elizabeth (b. 1635), Anne (b. 1637) and Katherine (b. 1639) - are generally either forgotten or ignored by history.
However, Elizabeth and Anne were reportedly both killed by their frail constitutions, while Katherine is according to various sources either listed as a stillborn child; born alive but died immediately after birth; or lived only a day. Elizabeth was a relatively important figure in her short life, since at one point, Charles wanted to swap her out with Mary to marry the Prince of Orange, due to him considering the house of Nassau unworthy of a king's eldest daughter. Unfortunately, Lizzy died at Carisbrooke Castle on the Isle of Wight in 1650, a few days before the permission came for her to join her sister, Mary, in the Netherlands.
Employing some handwavium, say Elizabeth, Anne and Katherine had survived, and their health is somewhat better, how might these princesses grow up, where do they marry (if they marry at all), and do the younger two fall-in with the Catholicism of the French court? or do they join Elizabeth and the duke of Gloucester in refusing to become Catholic? And what effects does this have on England's future?