Emilie Richards is recommended by my mother, and I’ve read some of hers and enjoyed them as well. But I haven’t read any of her stuff since she stopped writing for Silhouette.
Susan Elizabeth Phillips is another that my mother praises for writing pure romance, that’s also fun and funny.
A one-off that is highly popular and features a great set of characters is Audrey Niffenegger’s The Time Traveler’s Wife. I have problems with some of the SF aspects of the book, but that doesn’t keep me from recognizing it’s a hugely entertaining work, and well worth the read.
Diana Galbadon has a number of fans for her works, beginning with Outlander. This is another work that pushes the boundaries of genre fiction - it’s considered a romance for very good reason, but it also involves time travel where a modern woman ends up falling into medieval Scotland and falling in love with a border lord.
If your MIL is fond of Nora Roberts she might also enjoy the mysteries of J.D. Robb - who is one of Nora Roberts’ nom de plume. These are near future mysteries with a strong dose of romance involved. I like Nora Roberts’ characters but can’t read them, for much the same reason that The Time Traveler’s Wife left a bad taste for me. If your MIL isn’t familiar with common SF tropes that will not likely be a problem for her.
Moving outside the genre you’ve mentions a couple of authors with extensive series that have been known to appeal to romance readers - based on being positively reviewed in Romantic Times. You’ll have to judge whether any of these would appeal to your MIL.
Lois McMaster Bujold’s Vorkosigan books, beginning with the omnibus edition of Cordelia’s Honor. It starts as pretty normal seeming space opera, and ends up becoming a wonderfully dark and moving romance, with politics.
Lindsay Davis has a series of highly entertaining, and informative, detective novels set in Flavian Rome, that involve some wonderful romance, and relationships. The first book in the M. Didius Falco books is The Silver Pigs, and I cannot recommend it enough.
Janet Evanovich has taken on the stereotypes of the modern detective/mystery novel, and kicked them in the groin for much enjoyable fun. Her Stephanie Plum books are great fun, (though after about the tenth one they start to lose steam) and begin with One for the Money
Diane Mott Davidson has a series of mystery books featuring her amateur sleuth Goldy Baer, who’s trying to run a catering business, but for the bodies… The first book in this series is Catering to Nobody.
Finally, a bit harder to find, and a bit dated, now, Charlotte MacLeod did a good number of so-called screwball mysteries that were popular with many people I know who read romances. She had two or three main series that I can recall: Her Professor Shandy books, set at a fictional Massachusetts Agricultural college, and her Sarah Kelling/Max Bittersohn books set against the backdrop of Boston so-called high society. The first book in the Prof. Shandy books is Rest You Merry (and features one of the best gotcha’s for Christmas decorating maniacs I’ve ever had the pleasure to read.). The first book in the Sarah Kelling/Max Bittersohn books is The Family Vault