A man once entered the mosque where the prophet -pbuh- was sitting, prayed, then gave Salam to the prophet, who told him "Go back and Pray, for you have not prayed". The man thought it was strange but obeyed the prophet. He did it the same way, when he was done the prophet said "Go back and Pray, for you have not prayed".He did it a third time, at the end of which the prophet again said "Go back and Pray, for you have not prayed", to which the man replied "By God who sent you with the Truth I don't know how to pray better, please teach me." The prophet told him "If you stand to pray, make your takbeer and then recite what you comes easy of the Quran, Bow and wait till you rest bowing, then stand and wait till you rest standing, then prostate and wait till you rest prostating, then sit and wait till you rest sitting, then prostate and wait till you rest prostating,and do that in your whole prayer." (Muttafaqon Alaih both Moslem and Bukhary)
The prophet also said: "The worst of thieves are those who steal from their prayer," to which he was asked by the companions how could someone steal from their prayer? He answered: "They would not fulfill its bow and prostration." In another hadith the prophet said that whomever prays the way a crow picks at the ground, will not have it accepted, and in a version he said the prayer will not rise to God, but will be taken by an angel and sent down to its performer, slapped across his face with it, and the prayer will say "May you be wasted and lost the way you wasted and lost me!" and he will get no reward for it.
Sheikh Albani explained that the fajr prayer the prophet used to conduct it sometimes with the whole of Al-Baqara chapter till the companions used to fear the sun would rise upon them, and that today's prayers are short and fast already as it is. He insists that appropriate recitation and slow deliberate ritual movement is necessary and an integral part of the prayer.
In his book "The prayer of the prophet" he mentions that every bow and rise and every prostration and sitting need to be done slowly and for the person to wait until every bone, muscle, and joint falls into place and relaxes in that position without any muscles being tightly held in anticipation of the next movement. They also are the same length as each other, the bow is as long as the stand as long as the prostration as long as the sitting.
The reciting of the quran needs to be as slow and pronounced as you would be reciting legibly to a mass of people who are listening to what you are saying for the first time. The pause between every verse equivelent length to each other and sufficient for a separate breath. When bowing, "Subhanaka Rabbeya Al-Atheem" is recited slowly as reciting the quran while standing, and the minimum is one and can be said in odd numbers till 9 (some scholars say 10 or 11 is also sunnah). Same is with "Subhanaka Rabbeya Al-Aala".
Those speeds do not change whether you are reciting loudly or quietly to yourself, and your mouth needs to always move with the words pronounced properly.
As for telling your cousins, Sari, remember that we are muslims, described by God in the Quran "[FONT=Verdana,arial]Ye are the best of peoples, evolved for mankind, enjoining what is right, forbidding what is wrong, and believing in Allah." [/FONT](3:110), so it is our obligation to enjoin what is right and good and forbid what is wrong and evil, and this is one of the greatest things that need to teach those who don't know. So you need to tell them what you know is right, and to tell them off what they do that you know is wrong. Use kindness and education rather than denouncing otherwise they will be indignant or insulted and would rather ignore you than listen to you. Go online and find more proofs and details on how a slow prayer needs to be and guide them, may you be rewarded for their improvement. Remember though if they resist and refuse, be patient and let them be at least for a while, it is our responsibility to tell and inform, not enforce. It is their responsibility and their prayer.
Barak Allahu Feekee Wa Jazakee Allahu Khairan