Hello, I'm Dr. Bob.
I'm sorry you are disappointed with the clipped wings of your lovebirds. This is often done with young birds to facilitate taming, and to prevent their escape from unfamiliar new homes.
Normally, the feathers will not regrow until their first molt, usually this takes place at from 4 to 8 months of age, most commonly at 4 to 6 months.
You can speed up their regrowth by plucking out the shafts of the clipped feathers. But this is not a pleasant task, and must be done, two feathers at a time, over a period of several days. Their little wings are delicate and easily injured, and this uncomfortable procedure definitely would not endear you to your new pets, so I don't recommend doing it yourself. A veterinarian with avian experience can do this for you, but it will probably require several trips to their office.
Proper diet, rest and environment can help speed up the time of molting, as well as help them to develop properly.
Compare what you're feeding them to what professional aviary keepers feed their parrots: 70% high quality pelleted small parrot food, such as those made by Kaytee, Zupreem, Harrison's, Lafeber, or other reputable manufacturers (these may be purchased online), 20% dark leafy greens like kale, spinach, Swiss chard, cilantro, collard, turnip or beet greens, cooked sweet potato, squash, or pumpkin, carrots with the tops attached, dried chili pepper, yellow, green and red fresh peppers, and only 10% seeds and nut foods. Lettuce is practically all water,is of no food value in the diet, and shouldn't be fed to birds. Fruit is okay as an occasional treat, but also contains 90% water and fills the bird up without providing many nutrients by weight. Grit is no longer considered necessary to the healthy upkeep of pet birds, and grit impactions can be fatal. Most pet birds' diets consist of too high a proportion of seeds and nuts, which are out of balance in calcium and phosphorus, too high in fat, and low in iodine and this imbalance eventually catches up with them. Sunflower seeds are especially high in fat content. Beside all this, bird food is usually not shelf dated, and is often more than a year old when purchased. Most of the perishable nutrients are often gone by the time the birds actually eat it. Seeds and nuts tend to go rancid in time, and if the birds don't refuse to eat them, oil soluble vitamin deficiencies may result. Check your bird food package to see if there is a "Best by" date printed on the label. You can tell a lot about the manufacturer by whether or not they date their product, in spite of what they say about themselves in advertising or on the package. Proper nutrients in balance with one another help prevent digestive, respiratory, nerve, skin, feather plucking and organ problems, as well as behavior problems like self mutilation, excessive vocalization, aggression and excessive egg laying by females. One of the major reasons for this is that diets high in seed and nut foods tend to cause the liver to become sick, as fat tissue replaces healthy liver tissue. Excessive fats in seed-rich diets encourage the overproduction of sexual hormones causing moodiness, and unwanted sexual behavior as well as other side-effects. Although it may not be easy to switch to a better diet, even at their young age, you can read an excellent article about how to get them to eat more properly online here: http://www.cockatielcottage.net/finicky.html A properly balanced diet will optimize their growth and development.
Cockatiels kept as pets in the average home live an average of seven years, while those kept in professionally maintained aviaries routinely live as long as 35 years, the difference is primarily dietary. The statistics would be proportionately similar for your lovebirds.
Another consideration is proper rest. Birds should be allowed 12-14 hours of sleep under a dark cover in a quiet part of the house, away from T.V. radio or conversational noises.
Environment also plays a part to help build them up emotionally, and improvements help them to deal with stress more effectively. New toys, changed frequently to combat boredom, can be helpful, and you may want to consider placing natural tree branches in their cage for the distraction, interest, and safe chewing exercise. Be sure to use non-toxic branches only, a list can be found here: www.mdvaden.com/bird_page.shtml This is called "environmental enrichment" and is often helpful in stimulating, entertaining, and distracting them from bad habits and moods, which in turn, is good for their overall health.
I would suggest using the time until their feathers regrow to work with them daily to encourage them to bond with you. When they can simply fly away, it is distracting and very difficult to establish a relationship with them.
If you should have further questions, please let me know.
Best regards,
Dr. Bob

Hello again, sorry for the delay in getting back to you, I've been in and out all afternoon and just noticed your reply.
Birds, like other intelligent animals, are a bit unpredictable. They will most likely get along just fine, but if they are brother and sister, they should not be bred together. More distant relatives may produce normal offspring, but the chances of genetic problems in the chicks would be many time greater for sibling crossings. They do not have any natural aversion to "incest", and if they should produce chicks, they may actually be normal, but the odds would not be in their favor.
I hope that is what you meant by your question, but if you still have questions, I'm here.
Dr. Bob